Emit Kafka exception when err callback used

Previously Kafka dropped the exception and just incremented using the
same callback our wait. This will now print the exception message
associated, so we can distinguish between a successful and failed
callback in unit test logs.

I could have made it so that the exception is thrown in the main thread
by taking a s_ptr. However it quickly becomes unweildly for a simple
probe, and we only gain the stack trace - which was already lost when
extractData re throws the exception.