Add stack traces to logging

[plbertrand]

Also, small tip (which applies to amqtt in general too, I see the pattern there in lots of places):

            try:
                # … etc …
                pass
            except ConnectException as ce:
                logger.error("Connection failed: %s" % ce)

Not the best way to log an exception for two reasons:

1. % always formats the string, whether logger is configured to emit logs at error level or not
2. you miss out on the stack trace, which can be real handy for troubleshooting

logger.error("Connection failed: %s", ce) addresses (1) whilst ignoring (2). The logging module has the exc_info keyword argument, which pretty much will do this for you… I'd write the above as:

            try:
                # … etc …
                pass
            except ConnectException:
                logger.error("Connection failed", exc_info=True)

Or, for error, there's a shortcut:

            try:
                # … etc …
                pass
            except ConnectException:
                logger.exception("Connection failed")

Originally posted by @sjlongland in #75 (comment)