don't allow get_offset_manager to return None silently. Fixes #402'
don't allow get_offset_manager to return None silently. Fixes #402'

wikimedia/operations-debs-python-pykafka,thedrow/samsa,benauthor/pykafka,wikimedia/operations-debs-python-pykafka,wikimedia/operations-debs-python-pykafka,thedrow/samsa,benauthor/pykafka,thedrow/samsa,benauthor/pykafka