[SPARK-21503][UI] Spark UI shows incorrect task status for a killed Executor Process

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.

core/src/main/scala/org/apache/spark/ui/exec/ExecutorsTab.scala

@@ -19,7 +19,7 @@ package org.apache.spark.ui.exec
 
 import scala.collection.mutable.{LinkedHashMap, ListBuffer}
 
-import org.apache.spark.{ExceptionFailure, Resubmitted, SparkConf, SparkContext}
+import org.apache.spark.{Resubmitted, SparkConf, SparkContext}
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.scheduler._
 import org.apache.spark.storage.{StorageStatus, StorageStatusListener}
@@ -131,17 +131,17 @@ class ExecutorsListener(storageStatusListener: StorageStatusListener, conf: Spar
     if (info != null) {
       val eid = info.executorId
       val taskSummary = executorToTaskSummary.getOrElseUpdate(eid, ExecutorTaskSummary(eid))
-      taskEnd.reason match {
-        case Resubmitted =>
-          // Note: For resubmitted tasks, we continue to use the metrics that belong to the
-          // first attempt of this task. This may not be 100% accurate because the first attempt
-          // could have failed half-way through. The correct fix would be to keep track of the
-          // metrics added by each attempt, but this is much more complicated.
-          return
-        case _: ExceptionFailure =>
-          taskSummary.tasksFailed += 1
-        case _ =>
-          taskSummary.tasksComplete += 1
+      // Note: For resubmitted tasks, we continue to use the metrics that belong to the
+      // first attempt of this task. This may not be 100% accurate because the first attempt
+      // could have failed half-way through. The correct fix would be to keep track of the
+      // metrics added by each attempt, but this is much more complicated.
+      if (taskEnd.reason == Resubmitted) {
+        return
+      }
+      if (info.successful) {
+        taskSummary.tasksComplete += 1
+      } else {
+        taskSummary.tasksFailed += 1
       }
       if (taskSummary.tasksActive >= 1) {
         taskSummary.tasksActive -= 1