[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.
[SPARK-2318] When exiting on a signal, print the signal name first.

Author: Reynold Xin <rxin@apache.org>

Closes #1260 from rxin/signalhandler1 and squashes the following commits:

8e73552 [Reynold Xin] Uh add Logging back in ApplicationMaster.
0402ba8 [Reynold Xin] Synchronize SignalLogger.register.
dc70705 [Reynold Xin] Added SignalLogger to YARN ApplicationMaster.
79a21b4 [Reynold Xin] Added license header.
0da052c [Reynold Xin] Added the SignalLogger itself.
e587d2e [Reynold Xin] [SPARK-2318] When exiting on a signal, print the signal name first.