Make cros login error message more descriptive.

tools/telemetry/telemetry/core/chrome/cros_util.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.
 
 from telemetry.core import exceptions
 from telemetry.core import util
 
 def _SigninUIState(oobe):
   """Returns the signin ui state of the oobe. HIDDEN: 0, GAIA_SIGNIN: 1,
   ACCOUNT_PICKER: 2, WRONG_HWID_WARNING: 3, MANAGED_USER_CREATION_FLOW: 4.
@@ skipping 58 matching lines @@
   """Navigates through oobe login screen as guest"""
   oobe = browser_backend.misc_web_contents_backend.GetOobe()
   assert oobe
   WaitForAccountPicker(oobe)
   _ClickBrowseAsGuest(oobe)
   WaitForGuestFsMounted(cri)
 
 def NavigateLogin(browser_backend):
   """Navigates through oobe login screen"""
   # Dismiss the user image selection screen.
-  util.WaitFor(lambda: _WebContentsNotOobe(browser_backend), 15)
+  try:
+    util.WaitFor(lambda: _WebContentsNotOobe(browser_backend), 15)
+  except util.TimeoutException:
+    raise exceptions.LoginException(
+        'Timed out going through oobe screen. Make sure the custom auth '
+        'extension passed through --auth-ext-path is valid and has belongs '

[achuithb, 2013/05/23 22:36:55]

remove 'has'

[Tim Song, 2013/05/23 23:02:33]

Done.

+        'to user "chronos".')
 
   # Wait for the startup window, then close it.
   util.WaitFor(lambda: _StartupWindow(browser_backend) is not None, 20)
   _StartupWindow(browser_backend).Close()