[SPARK-2284][UI] Mark all failed tasks as failures.

Previously only tasks failed with ExceptionFailure reason was marked as failure.

Author: Reynold Xin <rxin@apache.org>

Closes #1224 from rxin/SPARK-2284 and squashes the following commits:

be79dbd [Reynold Xin] [SPARK-2284][UI] Mark all failed tasks as failures.

(cherry picked from commit 4a346e242c3f241c575f35536220df01ad724e23)
Signed-off-by: Reynold Xin <rxin@apache.org>