[SPARK-12059][CORE] Avoid assertion error when unexpected state transition met in Master

Downgrade to warning log for unexpected state transition.

andrewor14 please review, thanks a lot.

Author: jerryshao <sshao@hortonworks.com>

Closes #10091 from jerryshao/SPARK-12059.

Author: jerryshao

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -257,8 +257,9 @@ private[deploy] class Master(
           exec.state = state
 
           if (state == ExecutorState.RUNNING) {
-            assert(oldState == ExecutorState.LAUNCHING,
-              s"executor $execId state transfer from $oldState to RUNNING is illegal")
+            if (oldState != ExecutorState.LAUNCHING) {
+              logWarning(s"Executor $execId state transfer from $oldState to RUNNING is unexpected")
+            }
             appInfo.resetRetryCount()
           }