[SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error

The spark.local.dir is configured as a list of multiple paths as follows /data1/sparkenv/local,/data2/sparkenv/local. If the disk data2 of the driver node has error, the application will exit since DiskBlockManager exits directly at createLocalDirs. If the disk data2 of the worker node has error, the executor will exit either.
DiskBlockManager should not exit directly at createLocalDirs if one of spark.local.dir has error. Since spark.local.dir has multiple paths, a problem should not affect the overall situation.
I think DiskBlockManager could ignore the bad directory at createLocalDirs.

Author: yantangzhai <tyz0303@163.com>

Closes #1274 from YanTangZhai/SPARK-2324 and squashes the following commits:

609bf48 [yantangzhai] [SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error
df08673 [yantangzhai] [SPARK-2324] SparkContext should not exit directly when spark.local.dir is a list of multiple paths and one of them has error
[SPARK] Fix NPE for ExternalAppendOnlyMap

It did not handle null keys very gracefully before.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1288 from andrewor14/fix-external and squashes the following commits:

312b8d8 [Andrew Or] Abstract key hash code
ed5adf9 [Andrew Or] Fix NPE for ExternalAppendOnlyMap