[SPARK-7554] [STREAMING] Throw exception when an active/stopped...

[SPARK-7554] [STREAMING] Throw exception when an active/stopped StreamingContext is used to create DStreams and output operations

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

Closes #6099 from tdas/SPARK-7554 and squashes the following commits:

2cd4158 [Tathagata Das] Throw exceptions on attempts to add stuff to active and stopped contexts.