Poll error callbacks

[ignytis]

Hello,

This PR adds a callback logic for errors occurred on poll method call in producer. The idea is to handle several kinds of errors immediately.

Here is a use case for this feature. If the application fails to connect or to authenticate to Kafka server then it exits with error code:
https://github.com/ignytis/send2kafka/blob/705f022beb50d8c8be77abbf6e15bd79707e9ebe/src/kafka_producer.rs#L32 <- setting the callback
https://github.com/ignytis/send2kafka/blob/705f022beb50d8c8be77abbf6e15bd79707e9ebe/src/kafka_producer.rs#L8-L13 <- callback itself
This way the connection issues might be handled before trying to send actual messages to the broker.

I'd appreciate any recommendations if there are any option to improve this code or implement this feature the other way.