The executor tab on Spark UI page shows task as completed when an executor process that is running that task is killed using the kill command.
Added the case ExecutorLostFailure which was previously not there, thus, the default case would be executed in which case, task would be marked as completed. This case will consider all those cases where executor connection to Spark Driver was lost due to killing the executor process, network connection etc.

## How was this patch tested?
Manually Tested the fix by observing the UI change before and after.
Before:
<img width="1398" alt="screen shot-before" src="https://user-images.githubusercontent.com/22228190/28482929-571c9cea-6e30-11e7-93dd-728de5cdea95.png">
After:
<img width="1385" alt="screen shot-after" src="https://user-images.githubusercontent.com/22228190/28482964-8649f5ee-6e30-11e7-91bd-2eb2089c61cc.png">

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: pgandhi <pgandhi@yahoo-inc.com>
Author: pgandhi999 <parthkgandhi9@gmail.com>

Closes #18707 from pgandhi999/master.

(cherry picked from commit f016f5c8f6c6aae674e9905a5c0b0bede09163a4)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>