Driver process hangs when an exception occurs

[travisdowns]

If an exception in the driver during a run (e.g., some sendPost method fails, e.g., because the configuration was invalid) the driver will hang, at least when using multiple distributed workers (DistributedWorkersEnsemble).

The reason is that the worker.close() call will be skipped over if an exception is thrown during the run. This leaves the AsyncHttpClient object unclosed, and holding on to background threads which will hangs process on shutdown (as these are not daemon threads).