[SPARK-7736] [CORE] [YARN] Make pyspark fail YARN app on failure.

The YARN backend doesn't like when user code calls `System.exit`,
since it cannot know the exit status and thus cannot set an
appropriate final status for the application.

So, for pyspark, avoid that call and instead throw an exception with
the exit code. SparkSubmit handles that exception and exits with
the given exit code, while YARN uses the exit code as the failure
code for the Spark app.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7751 from vanzin/SPARK-9416.