@Override
    public void onBarrierError(String version, Throwable t) {
      LOG.error("Encountered error while attaining consensus on JobModel version " + version);
      metrics.barrierError.inc();
      stop();
    }
Builds the {@link GrouperMetadataImpl} based upon provided {@param jobModelVersion}
and {@param processorNodes}.
@param jobModelVersion the most recent jobModelVersion available in the zookeeper.
@param processorNodes the list of live processors in the zookeeper.
@return the built grouper metadata.
samza-core/src/main/java/org/apache/samza/zk/ZkJobCoordinator.java