Document how to add callbacks/errbacks to KafkaProducer message Futures

[jeffwidman]

We should document how to do this: https://stackoverflow.com/questions/46388116/how-to-add-a-failure-callback-for-kafka-python-kafka-kafkaproducersend

If my answer is amiss at all, please feel free to correct it.

[jeffwidman]

Fixed in #1441