Send the quit message and close the socket.

        See `Connection.close() <https://www.python.org/dev/peps/pep-0249/#Connection.close>`_
        in the specification.

        :raise Error: If the connection is already closed.

def close(self):
        """
        Send the quit message and close the socket.

        See `Connection.close() <https://www.python.org/dev/peps/pep-0249/#Connection.close>`_
        in the specification.

        :raise Error: If the connection is already closed.
        """
        if self._closed:
            raise err.Error("Already closed")
        self._closed = True
        if self._sock is None:
            return
        send_data = struct.pack('<iB', 1, COMMAND.COM_QUIT)
        try:
            self._write_bytes(send_data)
        except Exception:
            pass
        finally:
            self._force_close()