[SPARK-36614][CORE][UI] Correct executor loss reason caused by decomm…

…ission in UI

### What changes were proposed in this pull request?

Post the correct executor loss reason to UI.

### Why are the changes needed?

To show the accurate loss reason.

### Does this PR introduce _any_ user-facing change?

Yes. Users can see the difference from the UI.

Before:
<img width="509" alt="WeChataad8d1f27d9f9aa7cf93ced4bcc820e2" src="https://user-images.githubusercontent.com/16397174/131341692-6f412607-87b8-405e-822d-0d28f07928da.png">
<img width="1138" alt="WeChat13c9f1345a096ff83d193e4e9853b165" src="https://user-images.githubusercontent.com/16397174/131341699-f2c9de09-635f-49df-8e27-2495f34276c0.png">

After:

<img width="599" alt="WeChata4313fa2dbf27bf2dbfaef5c1d4a19cf" src="https://user-images.githubusercontent.com/16397174/131341754-e3c93b5d-5252-4006-a4cc-94d76f41303b.png">
<img width="1182" alt="WeChat5559d52fd3070ae6c42fe32d56f9dc94" src="https://user-images.githubusercontent.com/16397174/131341761-e1e0644f-1e76-49c0-915a-26aad77ec272.png">

### How was this patch tested?

Manully tested.

Closes apache#33868 from Ngone51/fix-executor-remove-reason.

Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit ebe7bb6)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -409,8 +409,8 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, val rpcEnv: Rp
           totalCoreCount.addAndGet(-executorInfo.totalCores)
           totalRegisteredExecutors.addAndGet(-1)
           scheduler.executorLost(executorId, lossReason)
-          listenerBus.post(
-            SparkListenerExecutorRemoved(System.currentTimeMillis(), executorId, reason.toString))
+          listenerBus.post(SparkListenerExecutorRemoved(
+            System.currentTimeMillis(), executorId, lossReason.toString))
         case None =>
           // SPARK-15262: If an executor is still alive even after the scheduler has removed
           // its metadata, we may receive a heartbeat from that executor and tell its block