code: def error(self):
          
          status_code, error_msg, payload = self.check_error()
  
          if status_code != 200 and not error_msg and not payload:
              return "Async error (%s). Status code: %r" % (self.async_id, status_code)
          return error_msg
docs: Check if the async response is an error.
  
          Take care to call `is_done` before calling `error`. Note that the error
          messages are always encoded as strings.
  
          :raises CloudUnhandledError: When not checking `is_done` first
          :return: the error value/payload, if found.
          :rtype: str