Improve error messages from checkperms PRESUBMIT check

PRESUBMIT.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 638 matching lines @@
 
 
 def _CheckFilePermissions(input_api, output_api):
   """Check that all files have their permissions properly set."""
   if input_api.platform == 'win32':
     return []
   args = [input_api.python_executable, 'tools/checkperms/checkperms.py',
           '--root', input_api.change.RepositoryRoot()]
   for f in input_api.AffectedFiles():
     args += ['--file', f.LocalPath()]
-  checkperms = input_api.subprocess.Popen(args,
-                                          stdout=input_api.subprocess.PIPE)
-  errors = checkperms.communicate()[0].strip()
-  if errors:
-    return [output_api.PresubmitError('checkperms.py failed.',
-                                      errors.splitlines())]
-  return []
+  try:
+    input_api.subprocess.check_output(args)
+    return []
+  except input_api.subprocess.CalledProcessError as error:
+    return [output_api.PresubmitError(
+        'checkperms.py failed:',
+        long_text=error.output)]
 
 
 def _CheckNoAuraWindowPropertyHInHeaders(input_api, output_api):
   """Makes sure we don't include ui/aura/window_property.h
   in header files.
   """
   pattern = input_api.re.compile(r'^#include\s*"ui/aura/window_property.h"')
   errors = []
   for f in input_api.AffectedFiles():
     if not f.LocalPath().endswith('.h'):
@@ skipping 1253 matching lines @@
   for master in masters:
     try_config.setdefault(master, {})
     for builder in masters[master]:
       # Do not trigger presubmit builders, since they're likely to fail
       # (e.g. OWNERS checks before finished code review), and we're
       # running local presubmit anyway.
       if 'presubmit' not in builder:
         try_config[master][builder] = ['defaulttests']
 
   return try_config