A developer encountered an AccessDenied exception today on a Mac

Catch AccessDenied error in two other places in process_manager.py

Testing Done:
A developer encountered an AccessDenied exception today on a Mac

```
 File "/Users/pantsman/.pex/install/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl.d5493d74c62f5a977c5119c97760061ccd69069a/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl/pants/java/nailgun_executor.py", line 180, in _get_nailgun_client
    running, updated = self._check_nailgun_state(new_fingerprint)
  File "/Users/pantsman/.pex/install/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl.d5493d74c62f5a977c5119c97760061ccd69069a/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl/pants/java/nailgun_executor.py", line 169, in _check_nailgun_state
    old_fp=self.fingerprint, new_fp=new_fingerprint,
  File "/Users/toby/.pex/install/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl.d5493d74c62f5a977c5119c97760061ccd69069a/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl/pants/java/nailgun_executor.py", line 111, in fingerprint
    if self.cmdline:
  File "/Users/pantsman/.pex/install/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl.d5493d74c62f5a977c5119c97760061ccd69069a/pantsbuild.pants-0.0.49_square_20150924_01-py2-none-any.whl/pants/pantsd/process_manager.py", line 97, in cmdline
    return process.cmdline()
  File "/Users/pantsman/.pex/install/psutil-3.1.1-py2.7-macosx-10.4-x86_64.egg.35468d99e847e571301799156d9472c88ad8b757/psutil-3.1.1-py2.7-macosx-10.4-x86_64.egg/psutil/__init__.py", line 584, in cmdline
    return self._proc.cmdline()
  File "/Users/pantsman/.pex/install/psutil-3.1.1-py2.7-macosx-10.4-x86_64.egg.35468d99e847e571301799156d9472c88ad8b757/psutil-3.1.1-py2.7-macosx-10.4-x86_64.egg/psutil/_psosx.py", line 210, in wrapper
    raise AccessDenied(self.pid, self._name)
Exception message: psutil.AccessDenied (pid=19752, name='hdiejectd')
```

A simpler patch (adding an extra catch of AccessDenied exception) solved the problem for the user.

Bugs closed: 2343

Reviewed at https://rbcommons.com/s/twitter/r/2951/

src/python/pants/pantsd/process_manager.py

@@ -22,29 +22,30 @@
 logger = logging.getLogger(__name__)
 
 
+@contextmanager
+def swallow_psutil_exceptions():
+  """A contextmanager that swallows standard psutil access exceptions."""
+  try:
+    yield
+  except (psutil.AccessDenied, psutil.NoSuchProcess):
+    # This masks common, but usually benign psutil process access exceptions that might be seen
+    # when accessing attributes/methods on psutil.Process objects.
+    pass
+
+
 class ProcessGroup(object):
   """Wraps a logical group of processes and provides convenient access to ProcessManager objects."""
 
   def __init__(self, name):
     self._name = name
 
-  @contextmanager
-  def _swallow_psutil_exceptions(self):
-    """A contextmanager that swallows standard psutil access exceptions."""
-    try:
-      yield
-    except (psutil.AccessDenied, psutil.NoSuchProcess):
-      # This masks common, but usually benign psutil process access exceptions that might be seen
-      # when accessing attributes/methods on psutil.Process objects.
-      pass
-
   def _instance_from_process(self, process):
     """Default converter from psutil.Process to process instance classes for subclassing."""
     return ProcessManager(name=process.name(), pid=process.pid, process_name=process.name())
 
   def iter_processes(self, proc_filter=None):
     proc_filter = proc_filter or (lambda x: True)
-    with self._swallow_psutil_exceptions():
+    with swallow_psutil_exceptions():
       for proc in (x for x in psutil.process_iter() if proc_filter(x)):
         yield proc
 
@@ -91,13 +92,10 @@ def cmdline(self):
 
     :returns: The command line or else `None` if the underlying process has died.
     """
-    try:
+    with swallow_psutil_exceptions():
       process = self._as_process()
       if process:
         return process.cmdline()
-    except psutil.NoSuchProcess:
-      # On some platforms, accessing attributes of a zombie'd Process results in NoSuchProcess.
-      pass
     return None
 
   @property
@@ -258,7 +256,7 @@ def is_alive(self):
       if (process.status() == psutil.STATUS_ZOMBIE or                    # Check for walkers.
           (self.process_name and self.process_name != process.name())):  # Check for stale pids.
         return False
-    except psutil.NoSuchProcess:
+    except (psutil.NoSuchProcess, psutil.AccessDenied):
       # On some platforms, accessing attributes of a zombie'd Process results in NoSuchProcess.
       return False
 

tests/python/pants_test/pantsd/test_process_manager.py

@@ -6,14 +6,13 @@
                         unicode_literals, with_statement)
 
 import os
-import subprocess
 import unittest
 from contextlib import contextmanager
 
 import mock
 import psutil
 
-from pants.pantsd.process_manager import ProcessGroup, ProcessManager
+from pants.pantsd.process_manager import ProcessGroup, ProcessManager, swallow_psutil_exceptions
 from pants.util.contextutil import temporary_dir
 
 
@@ -31,7 +30,7 @@ def setUp(self):
     self.pg = ProcessGroup('test')
 
   def test_swallow_psutil_exceptions(self):
-    with self.pg._swallow_psutil_exceptions():
+    with swallow_psutil_exceptions():
       raise psutil.NoSuchProcess('test')
 
   def test_iter_processes(self):