SPARK-1399: show stage failure reason in UI

https://issues.apache.org/jira/browse/SPARK-1399

refactor StageTable a bit to support additional column for failed stage

Author: CodingCat <zhunansjtu@gmail.com>
Author: Nan Zhu <CodingCat@users.noreply.github.com>

Closes #421 from CodingCat/SPARK-1399 and squashes the following commits:

2caba36 [CodingCat] remove dummy tag
77cf305 [CodingCat] create dummy element to wrap columns
3989ce2 [CodingCat] address Aaron's comments
18fc09f [Nan Zhu] fix compile error
00ea30a [Nan Zhu] address Kay's comments
16ac83d [CodingCat] set a default value of failureReason
35df3df [CodingCat] address andrew's comments
06d21a4 [CodingCat] address andrew's comments
25a6db6 [CodingCat] style fix
dc8856d [CodingCat] show stage failure reason in UI