Improve pyautolib errors.

chrome/test/pyautolib/pyauto_errors.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.
 
 """PyAuto Errors."""
 
 class JavascriptRuntimeError(RuntimeError):
   """Represent an error raised by injected Javascript."""
   pass
 
 
 class JSONInterfaceError(RuntimeError):
-  """Represent an error in the JSON ipc interface."""
+  """Represent an error in the JSON IPC interface."""
   pass
 
 
+class AutomationCommandFail(JSONInterfaceError):
+  """Represent an automation command failure.
+
+  These failures are passed back from the Chrome side of the IPC.
+  """
+  pass
+
+
 class NTPThumbnailNotShownError(RuntimeError):
   """Represent an error while attempting to manipulate a NTP thumbnail.
 
   This is due to it not being visible to a real user.
   """
   pass