Don't set librdkafka error_cb if a python callback is not provided.

[rnpridgeon]

#726 (comment)

[MSeal]

Copying fix context from linked issue:

I think we should close this PR and fix so that the Producer does not set the C error_cb unless a Py error_cb is specified. This way librdkafka will log broker failures by default, and it wont require the application to provide an error_cb, nor poll() to have it triggered.