Differentiate between temporary and permanent errors

From #128 comment by @davidblewett :
> I would appreciate updating the documentation as to what exceptions are transient and what are permanent. Even better, it would be very nice to encode that in the exception class hierarchy, so that people can have an exception handler for say KafkaTransientException that just logs it and moves on, and one for KafkaPermanentException that logs and triggers graceful shutdown.