mqtt-rpc-client: fix error response handling

[sikmir]

data is optional field in error response, see MQTT-RPC protocol.

Before:

$ mqtt-rpc-client -d wb-device-manager -s bus-scan -m Stop
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 3452, in _thread_main
    self.loop_forever(retry_first_connection=True)
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 1779, in loop_forever
    rc = self.loop(timeout, max_packets)
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 1181, in loop
    rc = self.loop_read(max_packets)
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 1572, in loop_read
    rc = self._packet_read()
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 2310, in _packet_read
    rc = self._packet_handle()
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 2936, in _packet_handle
    return self._handle_publish()
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 3216, in _handle_publish
    self._handle_on_message(message)
  File "/usr/lib/python3/dist-packages/paho/mqtt/client.py", line 3444, in _handle_on_message
    self.on_message(self, self._userdata, message)
  File "/usr/lib/python3/dist-packages/mqttrpc/client.py", line 96, in on_mqtt_message
    MQTTRPCError(result.error["message"], result.error["code"], result.error["data"])
KeyError: 'data'
Traceback (most recent call last):
  File "/usr/bin/mqtt-rpc-client", line 74, in <module>
    main()
  File "/usr/bin/mqtt-rpc-client", line 67, in main
    resp = rpc_client.call(
  File "/usr/lib/python3/dist-packages/mqttrpc/client.py", line 111, in call
    raise err
  File "/usr/lib/python3/dist-packages/mqttrpc/client.py", line 107, in call
    result = future.result(1e100 if timeout is None else timeout)
  File "/usr/lib/python3/dist-packages/mqttrpc/client.py", line 56, in result
    raise TimeoutError()
mqttrpc.client.TimeoutError

After:

$ mqtt-rpc-client -d wb-device-manager -s bus-scan -m Stop
Error: Task is already executing. [-33100]: None

[webconn]

Надо было codestyle в отдельном pr затронуть, но уже ладно