Failed StreamingContext.start() can leak active actors

StreamingContext.start() can throw exception because DStream.validateAtStart() fails (say, checkpoint directory not set for StateDStream). But by then JobScheduler, JobGenerator, and ReceiverTracker has already started, along with their actors. But those cannot be shutdown because the only way to do that is call StreamingContext.stop() which cannot be called as the context has not been marked as ACTIVE.