[SPARK-2286][UI] Report exception/errors for failed tasks that are not ExceptionFailure

Also added inline doc for each TaskEndReason.

Author: Reynold Xin <rxin@apache.org>

Closes #1225 from rxin/SPARK-2286 and squashes the following commits:

6a7959d [Reynold Xin] Fix unit test failure.
cf9d5eb [Reynold Xin] Merge branch 'master' into SPARK-2286
a61fae1 [Reynold Xin] Move to line above ...
38c7391 [Reynold Xin] [SPARK-2286][UI] Report exception/errors for failed tasks that are not ExceptionFailure.
(cherry picked from commit 6587ef7c1783961e6ef250afa387271a1bd6e277)

Conflicts:

	core/src/main/scala/org/apache/spark/ui/jobs/StageTable.scala