Factor workunit failure into final exit code.

Previously if a workunit fails, the run will end with Pants printing `FAILURE`, but the exit code is 0.

This change factors the result of workunit into the final exit code as well.

Before:
```bash
$ ./pants --pythonpath=tests/python --backend-packages=pants_test.goal.data do-some-work

23:41:53 00:00 [main]
               (To run a reporting server: ./pants server)
23:41:53 00:00   [setup]
23:41:53 00:00     [parse]
               Executing tasks in goals: bootstrap -> do-some-work
23:41:54 00:01   [bootstrap]
23:41:54 00:01     [substitute-aliased-targets]
23:41:54 00:01     [jar-dependency-management]
23:41:54 00:01     [bootstrap-jvm-tools]
23:41:54 00:01     [provide-tools-jar]
23:41:54 00:01   [do-some-work]
23:41:54 00:01     [do-some-work]
23:41:54 00:01       [non.existent.main.class]
                     ==== stderr ====
                     java.lang.ClassNotFoundException: non.existent.main.class
                     	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
                     	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
                     	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
                     	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
                     	at java.lang.Class.forName0(Native Method)
                     	at java.lang.Class.forName(Class.java:264)
                     	at com.martiansoftware.nailgun.NGSession.run(NGSession.java:242)

                     ==== stdout ====

23:41:54 00:01   [complete]
               FAILURE

$ echo $?
0
```

After:
```bash
$ ./pants --pythonpath=tests/python --backend-packages=pants_test.goal.data do-some-work

23:41:53 00:00 [main]
               (To run a reporting server: ./pants server)
23:41:53 00:00   [setup]
23:41:53 00:00     [parse]
               Executing tasks in goals: bootstrap -> do-some-work
23:41:54 00:01   [bootstrap]
23:41:54 00:01     [substitute-aliased-targets]
23:41:54 00:01     [jar-dependency-management]
23:41:54 00:01     [bootstrap-jvm-tools]
23:41:54 00:01     [provide-tools-jar]
23:41:54 00:01   [do-some-work]
23:41:54 00:01     [do-some-work]
23:41:54 00:01       [non.existent.main.class]
                     ==== stderr ====
                     java.lang.ClassNotFoundException: non.existent.main.class
                     	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
                     	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
                     	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
                     	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
                     	at java.lang.Class.forName0(Native Method)
                     	at java.lang.Class.forName(Class.java:264)
                     	at com.martiansoftware.nailgun.NGSession.run(NGSession.java:242)

                     ==== stdout ====

23:41:54 00:01   [complete]
               FAILURE

$ echo $?
1
```

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/161491405

Bugs closed: 3882

Reviewed at https://rbcommons.com/s/twitter/r/4244/

src/python/pants/bin/local_pants_runner.py

@@ -92,12 +92,14 @@ def _run(self):
                                        self._daemon_build_graph,
                                        self._exiter).setup()
 
-      result = goal_runner.run()
+      goal_runner_result = goal_runner.run()
 
       if repro:
         # TODO: Have Repro capture the 'after' state (as a diff) as well?
         repro.log_location_of_repro_file()
     finally:
-      run_tracker.end()
+      run_tracker_result = run_tracker.end()
 
-    self._exiter.exit(result)
+    # Take the exit code with higher abs value in case of negative values.
+    final_exit_code = goal_runner_result if abs(goal_runner_result) > abs(run_tracker_result) else run_tracker_result
+    self._exiter.exit(final_exit_code)

src/python/pants/goal/run_tracker.py

@@ -86,8 +86,8 @@ def __init__(self, *args, **kwargs):
     # run_id is safe for use in paths.
     millis = int((run_timestamp * 1000) % 1000)
     run_id = 'pants_run_{}_{}_{}'.format(
-               time.strftime('%Y_%m_%d_%H_%M_%S', time.localtime(run_timestamp)), millis,
-               uuid.uuid4().hex)
+      time.strftime('%Y_%m_%d_%H_%M_%S', time.localtime(run_timestamp)), millis,
+      uuid.uuid4().hex)
 
     info_dir = os.path.join(self.get_options().pants_workdir, self.options_scope)
     self.run_info_dir = os.path.join(info_dir, run_id)
@@ -317,6 +317,8 @@ def end(self):
     """This pants run is over, so stop tracking it.
 
     Note: If end() has been called once, subsequent calls are no-ops.
+
+    :return: 0 for success, 1 for failure.
     """
     if self._background_worker_pool:
       if self._aborted:
@@ -349,6 +351,8 @@ def end(self):
     self.report.close()
     self.store_stats()
 
+    return 1 if outcome in [WorkUnit.FAILURE, WorkUnit.ABORTED] else 0
+
   def end_workunit(self, workunit):
     self.report.end_workunit(workunit)
     path, duration, self_time, is_tool = workunit.end()

tests/python/pants_test/goal/data/register.py

@@ -0,0 +1,28 @@
+# coding=utf-8
+# Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from pants.backend.jvm.tasks.nailgun_task import NailgunTask
+from pants.base.workunit import WorkUnit
+from pants.goal.task_registrar import TaskRegistrar as task
+from pants.task.task import Task
+
+
+def register_goals():
+  task(name='run-dummy-workunit', action=TestWorkUnitTask).install()
+
+
+class TestWorkUnitTask(NailgunTask):
+  @classmethod
+  def register_options(cls, register):
+    register('--success', default=False, type=bool)
+
+  def execute(self):
+    result = WorkUnit.SUCCESS if self.get_options().success else WorkUnit.FAILURE
+
+    # This creates workunit and marks it as failure.
+    with self.context.new_workunit('dummy') as workunit:
+      workunit.set_outcome(result)

tests/python/pants_test/goal/test_run_tracker_integration.py

@@ -6,6 +6,7 @@
                         unicode_literals, with_statement)
 
 import json
+import os
 
 from pants.util.contextutil import temporary_file_path
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
@@ -27,3 +28,24 @@ def test_stats_local_json_file(self):
         self.assertIn('run_info', stats_json)
         self.assertIn('self_timings', stats_json)
         self.assertIn('cumulative_timings', stats_json)
+
+  def test_workunit_failure(self):
+    pants_run = self.run_pants([
+      '--pythonpath={}'.format(os.path.join(os.getcwd(), 'tests', 'python')),
+      '--backend-packages={}'.format('pants_test.goal.data'),
+      'run-dummy-workunit',
+      '--no-success'
+    ])
+    # Make sure the task actually happens and of no exception.
+    self.assertIn('[run-dummy-workunit]', pants_run.stdout_data)
+    self.assertNotIn('Exception', pants_run.stderr_data)
+    self.assert_failure(pants_run)
+
+  def test_workunit_success(self):
+    pants_run = self.run_pants([
+      '--pythonpath={}'.format(os.path.join(os.getcwd(), 'tests', 'python')),
+      '--backend-packages={}'.format('pants_test.goal.data'),
+      'run-dummy-workunit',
+      '--success'
+    ])
+    self.assert_success(pants_run)