Add 'mint_oauth_token' method to RPC client.

appengine/swarming/swarming_bot/bot_code/remote_client_errors.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 
 class InitializationError(Exception):
   """Raised by RemoteClient.initialize on fatal errors."""
   def __init__(self, last_error):
     super(InitializationError, self).__init__('Failed to grab auth headers')
     self.last_error = last_error
 
 
 class BotCodeError(Exception):
   """Raised by RemoteClient.get_bot_code."""
   def __init__(self, new_zip, url, version):
     super(BotCodeError, self).__init__(
         'Unable to download %s from %s; first tried version %s' %
         (new_zip, url, version))
 
 
 class InternalError(Exception):
   """Raised on unrecoverable errors that abort task with 'internal error'."""
 
 
 class PollError(Exception):
-  """Raised on unrecoverable errors when in RemoteClient.poll."""
+  """Raised on unrecoverable errors in RemoteClient.poll."""
+
+
+class MintOAuthTokenError(Exception):
+  """Raised on unrecoverable errors in RemoteClient.mint_oauth_token."""