Parses the content from a validator response Message.

def _parse_response(proto, response):
        """Parses the content from a validator response Message.
        """
        try:
            content = proto()
            content.ParseFromString(response.content)
            return content
        except (DecodeError, AttributeError):
            LOGGER.error('Validator response was not parsable: %s', response)
            raise errors.ValidatorResponseInvalid()