Add an actionable error message when image diff fails.

Tools/Scripts/webkitpy/layout_tests/port/port_testcase.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 210 matching lines @@
         exception_raised = False
         try:
             port.diff_image("EXPECTED", "ACTUAL")
         except ValueError, e:
             exception_raised = True
         self.assertFalse(exception_raised)
 
     def test_diff_image_crashed(self):
         port = self.make_port()
         port._executive = MockExecutive2(exit_code=2)
-        self.assertEqual(port.diff_image("EXPECTED", "ACTUAL"), (None, 'image diff returned an exit code of 2'))
+        self.assertEqual(port.diff_image("EXPECTED", "ACTUAL"), (None, 'Image diff returned an exit code of 2. See http://crbug.com/278596'))
 
     def test_check_wdiff(self):
         port = self.make_port()
         port.check_wdiff()
 
     def test_wdiff_text_fails(self):
         host = MockSystemHost(os_name=self.os_name, os_version=self.os_version)
         host.executive = MockExecutive(should_throw=True)
         port = self.make_port(host=host)
         port._executive = host.executive  # AndroidPortTest.make_port sets its own executive, so reset that as well.
@@ skipping 221 matching lines @@
         saved_environ = os.environ.copy()
         try:
             os.environ['WEBKIT_HTTP_SERVER_CONF_PATH'] = '/existing/httpd.conf'
             self.assertEqual(port.path_to_apache_config_file(), '/existing/httpd.conf')
         finally:
             os.environ = saved_environ.copy()
 
     def test_additional_platform_directory(self):
         port = self.make_port(options=MockOptions(additional_platform_directory=['/tmp/foo']))
         self.assertEqual(port.baseline_search_path()[0], '/tmp/foo')