Improve error messaging when ZooKeeper port conflicts block startup

When starting a cluster, if the ZooKeeper ports conflict (i.e. multiple ZK nodes running on the same physical hardware and attempting to use the same ports), the cluster will not start and the log will be flooded with error messages, making diagnosis difficult.

The initial error message should indicate that the ZK ports are identical and this may be the cause of the issue (NiFi doesn't know if the ZK hosts are on the same machine, but identical port numbers is suspicious), and the repeated log message should be vastly reduced in output rate.