Freshness tracker should fail a cluster iteration if all partitions for all consumers fails

[jyates]

Currently, we are very generous with the failure constraints for a cluster, from ConsumerFreshness (ln 281-293):

    // if all the consumer measurements succeed, then we return the cluster name
    // otherwise, Future.get will throw an exception representing the failure to measure a consumer (and thus the
    // failure to successfully monitor the cluster).
    return Futures.whenAllSucceed(completedConsumers).call(client::getCluster, this.executor);
  }

  /**
   * Measure the freshness for all the topic/partitions currently consumed by the given consumer group. To maintain
   * the existing contract, a consumer measurement fails ({@link Future#get()} throws an exception) only if:
   *  - burrow group status lookup fails
   *  - execution is interrupted
   * Failure to actually measure the consumer is swallowed into a log message & metric update; obviously, this is less
   * than ideal for many cases, but it will be addressed later.

However, SSL connection issues (i.e. a misconfiguration) only show up when querying the consumers. So you can have a valid burrow lookup for the cluster (b/c burrow is configured correctly) but freshness fails for each consumer because the tracker misconfigured. You would never know though (from the kafka_consumer_freshness_last_success_run_timestamp metric) since that will not get incremented for the failures.