[SPARK-10248][CORE] track exceptions in dagscheduler event loop in tests

`DAGSchedulerEventLoop` normally only logs errors (so it can continue to process more events, from other jobs).  However, this is not desirable in the tests -- the tests should be able to easily detect any exception, and also shouldn't silently succeed if there is an exception.

This was suggested by mateiz on https://github.com/apache/spark/pull/7699.  It may have already turned up an issue in "zero split job".

Author: Imran Rashid <irashid@cloudera.com>

Closes #8466 from squito/SPARK-10248.