Add an actionable error message when image diff fails.

Tools/Scripts/webkitpy/layout_tests/port/base.py

 # Copyright (C) 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 488 matching lines @@
         result = None
         err_str = None
         try:
             exit_code = self._executive.run_command(comand, return_exit_code=True)
             if exit_code == 0:
                 # The images are the same.
                 result = None
             elif exit_code == 1:
                 result = self._filesystem.read_binary_file(native_diff_filename)
             else:
-                err_str = "image diff returned an exit code of %s" % exit_code
+                err_str = "Image diff returned an exit code of %s. See http://crbug.com/278596" % exit_code
         except OSError, e:
             err_str = 'error running image diff: %s' % str(e)
         finally:
             self._filesystem.rmtree(str(tempdir))
 
         return (result, err_str or None)
 
     def diff_text(self, expected_text, actual_text, expected_filename, actual_filename):
         """Returns a string containing the diff of the two text strings
         in 'unified diff' format."""
@@ skipping 1233 matching lines @@
             if name.find('/') != -1:
                 _log.error("Virtual test suites names cannot contain /'s: %s" % name)
                 return
             self.name = 'virtual/' + name + '/' + base
         self.base = base
         self.args = args
         self.tests = tests or set()
 
     def __repr__(self):
         return "VirtualTestSuite('%s', '%s', %s)" % (self.name, self.base, self.args)