Add traceback for ConnectionError in ConsumerThread

[tefra]

Modified the exception handling in worker to include full traceback information for ConnectionError.

Resolves #756

[LincolnPuzey]

Looks sensible to me, thanks

[LincolnPuzey]

Actually after testing this, I am not in favor of adding the traceback.

Because it makes the logs from connection errors a lot noisier, and gives the impression of a bug in dramatiq, when connection errors are in fact known/expected errors.
Because starting dramatiq with no broker service running, is a likely scenario for a beginner, we want to avoid this.