Support error handling in RebalanceCallback

[nikitapecasa]

This is a follow-up task to #122.

Currently errors from consumer related methods are thrown in a poll thread,
and there's no way to provide recovering code inside [[RebalanceCallback]].

For example:

def onPartitionsAssigned(partitions: Nes[TopicPartition]) = {
  for {
    state <- lift(restoreStateFor(partitions))
    _       <- state.offsets.traverse_(o => seek(o.partition, o.offset))
  } yield ()
}

if seek operation fails, exception is thrown in a poll thread, and user cannot provide recovering code while building a RebalanceCallback.

To fix it we need to implement MonadThrowable instance for RebalanceCallback