CommsCallback catches throwables from MqttCallback - hard to use in tests

- [x] Bug exists Release Version 1.2.0 ( Master Branch)
- [x] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

Any throwably thrown by MqttCallback in the connectionLost callback method is caught in the CommsCallback class. This makes it hard to make a test fail if the connection is lost.

See:
https://github.com/eclipse/paho.mqtt.java/blob/6d7aafddb3a70d7c1eab69e1f9619af8f7ece47c/org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/internal/CommsCallback.java#L297

If this works as expected, do you have any recommendations for handling such a situation?

Start with org.eclipse.paho.client.mqttv3.internal/CommsCallback.java at the connectionLost handling around line 297, then reproduce the behavior with a callback that throws. Check how the callback exception is caught and determine how a test can observe the failure; done means the connection-lost callback behavior is testable without unexpectedly hiding the throwable.

Written by the indexing model from the issue text.