Propagate system broker errors into Hermann::Result objects

Provided that Producer#tick_reactor has been invoked, if there are underlying
errors like "1/1 Brokers down" then this code will ensure that outstanding
Herman::Result children of Herman::Producer will have their error status updated.

Fixes #25
Add a failing integration test for binary data

Just a quick reproduction case for #32
Merge pull request #33 from rtyler/issues/25-producer-errors

Propagate systemic errors from librdkafka up into Herman::Result objects