Improve error messages from checkperms PRESUBMIT check

third_party/WebKit/PRESUBMIT.py

 # Copyright (c) 2013 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 Blink.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 277 matching lines @@
     for line_num, line in f.ChangedContents():
         if pattern.match(line):
             errors.append('    %s:%d %s' % (f.LocalPath(), line_num, line))
     return errors
 
 
 def _CheckFilePermissions(input_api, output_api):
     """Check that all files have their permissions properly set."""
     if input_api.platform == 'win32':
         return []
-    path = input_api.os_path.join(
-        '..', '..', 'tools', 'checkperms', 'checkperms.py')
-    args = [sys.executable, path, '--root', input_api.change.RepositoryRoot()]
+    args = [input_api.python_executable,
+            input_api.os_path.join(
+                input_api.change.RepositoryRoot(),
+                '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:
+    try:
+        input_api.subprocess.check_output(args)
+        return []
+    except input_api.subprocess.CalledProcessError as error:
         return [output_api.PresubmitError(
-            'checkperms.py failed.', errors.splitlines())]
-    return []
+            'checkperms.py failed:',
+            long_text=error.output)]
 
 
 def _CheckForInvalidPreferenceError(input_api, output_api):
     pattern = input_api.re.compile('Invalid name for preference: (.+)')
     results = []
 
     for f in input_api.AffectedFiles():
         if not f.LocalPath().endswith('-expected.txt'):
             continue
         for line_num, line in f.ChangedContents():
@@ skipping 42 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