How to use the fbtftp.constants.OPCODE_ERROR function in fbtftp

def _transmit_error(self):
        """Transmits an error to the client and terminates the exchange."""
        fmt = str(
            "!HH%dsx" % (len(self._stats.error["error_message"].encode("latin-1")))
        )
        packet = struct.pack(
            fmt,
            constants.OPCODE_ERROR,
            self._stats.error["error_code"],
            bytes(self._stats.error["error_message"].encode("latin-1")),
        )
        self._get_listener().sendto(packet, self._peer)

"""
        # Note that we use blocking socket, because it has its own dedicated
        # process. We read only 512 bytes.
        try:
            listener = self._get_listener()
            listener.settimeout(self._timeout)
            data, peer = listener.recvfrom(constants.DEFAULT_BLKSIZE)
            listener.settimeout(None)
        except socket.timeout:
            return
        if peer != self._peer:
            logging.error("Unexpected peer: %s, expected %s" % (peer, self._peer))
            self._should_stop = True
            return
        code, block_number = struct.unpack("!HH", data[:4])
        if code == constants.OPCODE_ERROR:
            # When the client sends an OPCODE_ERROR#
            # the block number is the ERR codes in constants.py
            self._stats.error = {
                "error_code": block_number,
                "error_message": data[4:-1].decode("ascii", "ignore"),
            }
            # An error was reported by the client which terminates the exchange
            logging.error(
                "Error reported from client: %s" % self._stats.error["error_message"]
            )
            self._transmit_error()
            self._should_stop = True
            return
        if code != constants.OPCODE_ACK:
            logging.error(
                "Expected an ACK opcode from %s, got: %d" % (self._peer, code)