Handle an unknown RES command.

        The function makes sure to recv all message parts and respond with an
        error.

def _handle_unknown_command(self):
        """Handle an unknown RES command.

        The function makes sure to recv all message parts and respond with an
        error.

        """
        while self.query_socket.getsockopt(zmq.RCVMORE):
            # Making sure we 'empty' enveloped message. Otherwise, we can't
            # respond.
            self.query_socket.recv()
        self.query_socket.send(b"ERROR Unknown request type")