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.