[SPARK-2323] Exception in accumulator update should not crash DAGScheduler & SparkContext

Author: Reynold Xin <rxin@apache.org>

Closes #1772 from rxin/accumulator-dagscheduler and squashes the following commits:

6a58520 [Reynold Xin] [SPARK-2323] Exception in accumulator update should not crash DAGScheduler & SparkContext.