Add OAuth2 support for end users (i.e. 3-legged flow with the browser).

git_cl.py

 #!/usr/bin/env python
 # 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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld."""
 
 from distutils.version import LooseVersion
@@ skipping 3175 matching lines @@
     return 2
 
   # Reload settings.
   global settings
   settings = Settings()
 
   colorize_CMDstatus_doc()
   dispatcher = subcommand.CommandDispatcher(__name__)
   try:
     return dispatcher.execute(OptionParser(), argv)
+  except auth.AuthenticationError as e:
+    DieWithError(str(e))
   except urllib2.HTTPError, e:
     if e.code != 500:
       raise
     DieWithError(
         ('AppEngine is misbehaving and returned HTTP %d, again. Keep faith '
           'and retry or visit go/isgaeup.\n%s') % (e.code, str(e)))
   return 0
 
 
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   colorama.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)