[SPARK-7958] [STREAMING] Handled exception in StreamingContext.start() to prevent leaking of 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.

The solution in this PR is to stop the internal scheduler if start throw exception, and mark the context as STOPPED.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #6559 from tdas/SPARK-7958 and squashes the following commits:

20b2ec1 [Tathagata Das] Added synchronized
790b617 [Tathagata Das] Handled exception in StreamingContext.start()