code: def __format_error(self, error_list_tag):
      
      error = {'domain': self.domain(),
               'reason': self.reason(),
               'message': self.message()}
      error.update(self.extra_fields() or {})
      return {'error': {error_list_tag: [error],
                        'code': self.status_code(),
                        'message': self.message()}}
docs: Format this error into a JSON response.
  
      Args:
        error_list_tag: A string specifying the name of the tag to use for the
          error list.
  
      Returns:
        A dict containing the reformatted JSON error response.