Handle OffsetsCommitted consumer events
Since we're using enable.auto.commit, we'll receive OffsetsCommitted events to notify us of commit outcomes [0].
Explicitly handle these to avoid "Unexpected event type" log spam, only logging in the case where they're reporting an error.
[0] https://docs.confluent.io/platform/current/clients/confluent-kafka-go/index.html#hdr-Events