Better error messages needed when only 1 client worker deployed

[hcoyote]

03:20:20.240 [main] INFO - Using DistributedWorkersEnsemble workers topology
Exception in thread "main" java.lang.IllegalArgumentException
	at com.google.common.base.Preconditions.checkArgument(Preconditions.java:128)
	at io.openmessaging.benchmark.worker.DistributedWorkersEnsemble.<init>(DistributedWorkersEnsemble.java:71)
	at io.openmessaging.benchmark.Benchmark.main(Benchmark.java:158)

DistributedWorkersEnsemble throws this arg error if there happens to be only one configured worker.

Swarm at least throws a useful error message telling you it requires more than 1 worker.

03:27:43.539 [main] INFO - Using SwarmWorker workers topology
Exception in thread "main" java.lang.IllegalArgumentException: Workers must be > 1
	at com.google.common.base.Preconditions.checkArgument(Preconditions.java:142)
	at io.openmessaging.benchmark.worker.SwarmWorker.<init>(SwarmWorker.java:119)
	at io.openmessaging.benchmark.Benchmark.main(Benchmark.java:161)

TF should also probably throw an error if clients is less than 2