[SPARK-2322] Exception in resultHandler should NOT crash DAGScheduler and shutdown SparkContext.

This should go into 1.0.1.

Author: Reynold Xin <rxin@apache.org>

Closes #1264 from rxin/SPARK-2322 and squashes the following commits:

c77c07f [Reynold Xin] Added comment to SparkDriverExecutionException and a test case for accumulator.
5d8d920 [Reynold Xin] [SPARK-2322] Exception in resultHandler could crash DAGScheduler and shutdown SparkContext.

(cherry picked from commit 358ae1534d01ad9e69364a21441a7ef23c2cb516)
Signed-off-by: Reynold Xin <rxin@apache.org>

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala