Improve PantsRunnerTest setup error message.

This is in support of debugging OSX CI failures under Jenkins but is
also generally more friendly.

Testing Done:
Verified the capture code worked with:
```diff
$ git diff -U1
diff --git a/tests/java/org/pantsbuild/tools/runner/PantsRunnerTest.java b/tests/java/org/pantsbuild/tools/runner/PantsRunnerTest.java
index 864f096..271c4ab 100644
--- a/tests/java/org/pantsbuild/tools/runner/PantsRunnerTest.java
+++ b/tests/java/org/pantsbuild/tools/runner/PantsRunnerTest.java
@@ -83,3 +83,3 @@ public class PantsRunnerTest {
             stderr.readAll()),
-        0,
+        42,
         result);
```

Yielding:
```
$ ./pants test tests/java/org/pantsbuild/tools/runner:jar
...

16:00:23 00:03     [junit]
16:00:23 00:03       [run]
                     OpenJDK 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
                     OpenJDK 64-Bit Server VM warning: ignoring option UseSplitVerifier; support was removed in 8.0
                     Auto-detected 8 processors, using -parallel-threads=8
                     F
                     Time: 3.73
                     There was 1 failure:
                     1) org.pantsbuild.tools.runner.PantsRunnerTest
                     java.lang.AssertionError: Problem running ./pants bundle testprojects/src/java/org/pantsbuild/testproject/runner:pants-runner-testproject - exited with 0:
                     STDOUT:

                     16:00:26 00:00 [main]
                                    (To run a reporting server: ./pants server)
                     16:00:26 00:00   [setup]
                     16:00:26 00:00     [parse]
                                    Executing tasks in goals: bootstrap -> imports -> unpack-jars -> deferred-sources -> gen -> jvm-platform-validate -> resolve -> resources -> compile -> binary -> apk -> sign -> bundle
                     16:00:26 00:00   [bootstrap]
                     16:00:26 00:00     [substitute-aliased-targets]
                     16:00:26 00:00     [jar-dependency-management]
                     16:00:26 00:00     [bootstrap-jvm-tools]
                     16:00:26 00:00     [provide-tools-jar]
...
                     16:00:27 00:01   [complete]
                                    SUCCESS

                     STDERR:
                      expected:<42> but was:<0>
                     	at org.junit.Assert.fail(Assert.java:88)
                     	at org.junit.Assert.failNotEquals(Assert.java:834)
                     	at org.junit.Assert.assertEquals(Assert.java:645)
                     	at org.pantsbuild.tools.runner.PantsRunnerTest.setUpClass(PantsRunnerTest.java:77)
...
```

CI went green here:
  http://jenkins.pantsbuild.org/job/pantsbuild/job/pants/job/PR-3616/10/

Bugs closed: 3615, 3616

Reviewed at https://rbcommons.com/s/twitter/r/4038/

tests/java/org/pantsbuild/tools/runner/BUILD

@@ -5,6 +5,8 @@ junit_tests(
   name='jar',
   sources=globs('*.java'),
   dependencies=[
+    '3rdparty:guava',
+    '3rdparty:jsr305',
     '3rdparty:junit',
     'src/java/org/pantsbuild/tools/runner:runner-library',
   ],

tests/java/org/pantsbuild/tools/runner/PantsRunnerTest.java

@@ -3,12 +3,24 @@
 
 package org.pantsbuild.tools.runner;
 
-import java.io.*;
-import java.lang.String;
+import java.io.BufferedReader;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.PrintStream;
+import java.io.Reader;
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.concurrent.atomic.AtomicReference;
 
+import javax.annotation.Nullable;
+
+import com.google.common.base.Charsets;
+import com.google.common.io.CharStreams;
+import com.google.common.io.Closeables;
+
 import org.junit.BeforeClass;
 import org.junit.Test;
 
@@ -28,9 +40,54 @@ public class PantsRunnerTest {
           "testprojects.src.java.org.pantsbuild.testproject.runner.main-class-0.jar");
   static final String MAIN_CLASS = "org.pantsbuild.testproject.runner.MainClass";
 
+  static class StreamReader extends Thread {
+    static StreamReader start(InputStream stream) {
+      StreamReader reader = new StreamReader(stream);
+      reader.start();
+      return reader;
+    }
+
+    private final Reader reader;
+    @Nullable private String capturedData;
+
+    private StreamReader(InputStream stream) {
+      reader = new BufferedReader(new InputStreamReader(stream, Charsets.UTF_8));
+    }
+
+    @Override
+    public void run() {
+      try {
+        capturedData = CharStreams.toString(reader);
+      } catch (IOException e) {
+        throw new RuntimeException(e);
+      } finally {
+        Closeables.closeQuietly(reader);
+      }
+    }
+
+    @Nullable
+    String readAll() throws InterruptedException {
+      join();
+      return capturedData;
+    }
+  }
+
   @BeforeClass
-  public static void setup() throws Exception {
-    assertEquals(0, Runtime.getRuntime().exec("./pants bundle " + TEST_PROJECT).waitFor());
+  public static void setUpClass() throws Exception {
+    String command = "./pants bundle " + TEST_PROJECT;
+    Process process = Runtime.getRuntime().exec(command);
+    StreamReader stdout = StreamReader.start(process.getInputStream());
+    StreamReader stderr = StreamReader.start(process.getErrorStream());
+    int result = process.waitFor();
+    assertEquals(
+        String.format(
+            "Problem running %s - exited with %d:\nSTDOUT:\n%s\n\nSTDERR:\n%s",
+            command,
+            result,
+            stdout.readAll(),
+            stderr.readAll()),
+        0,
+        result);
     assertTrue(SYNTHETIC_JAR.exists());
     assertTrue(MAIN_JAR.exists());
   }