def reason(self):
raise NotImplementedError<EOL>
Get the reason for the error.

        Error reason is a custom string in the Cloud Endpoints server.  When
        possible, this should match the reason that the live server will generate,
        based on the error's status code.  If this returns None, the error formatter
        will attempt to generate a reason from the status code.

        Returns:
          None, by default.  Subclasses can override this if they have a specific
          error reason.