[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.

(cherry picked from commit 05bf4e4aff0d052a53d3e64c43688f07e27fec50)
Signed-off-by: Reynold Xin <rxin@apache.org>