Improve error when non-unique MQTT client ID causes broker disconnection

[ximon18]

Currently this results in the following unhelpful log message: (line wrapped for readability)

[2023-11-20 23:28:10] WARN  rotonda::targets::mqtt::status_reporter: mqtt: MQTT connection error: 
Mqtt state: Io error: Custom { kind: ConnectionAborted, error: "connection closed by peer" }

[ximon18]

At least in the case of broker.emqx.io it seems there's nothing we can do about this as the underlying rumqttc client code doesn't receive anything either, it just discovers that its connection has been closed hence the error message it produces. Also, when reconnecting we cause another connected client to disconnect, the rumqttc client code doesn't receive an error about duplicate client ID.

[ximon18]

The same behaviour was also observed with broker.hivemq.com, test.mosquitto.or, publicmqtt.bevywise.com and a locally running EMQX Docker MQTT broker.

[ximon18]

I wonder if this is an MQTTv3 protocol thing and with v5 it would be different?