[SPARK-14889][SPARK CORE] scala.MatchError: NONE (of class scala.Enumeration)...

[SPARK-14889][SPARK CORE] scala.MatchError: NONE (of class scala.Enumeration) when spark.scheduler.mode=NONE

## What changes were proposed in this pull request?

Handling exception for the below mentioned issue

```
➜  spark git:(master) ✗ ./bin/spark-shell -c spark.scheduler.mode=NONE
16/04/25 09:15:00 ERROR SparkContext: Error initializing SparkContext.
scala.MatchError: NONE (of class scala.Enumeration$Val)
	at org.apache.spark.scheduler.Pool.<init>(Pool.scala:53)
	at org.apache.spark.scheduler.TaskSchedulerImpl.initialize(TaskSchedulerImpl.scala:131)
	at org.apache.spark.SparkContext$.org$apache$spark$SparkContext$$createTaskScheduler(SparkContext.scala:2352)
	at org.apache.spark.SparkContext.<init>(SparkContext.scala:492)
```

The exception now looks like

```
java.lang.RuntimeException: The scheduler mode NONE is not supported by Spark.
```

## How was this patch tested?

manual tests

Author: Subhobrata Dey <sbcd90@gmail.com>

Closes #12666 from sbcd90/schedulerModeIssue.

core/src/main/scala/org/apache/spark/scheduler/Pool.scala

@@ -55,6 +55,8 @@ private[spark] class Pool(
         new FairSchedulingAlgorithm()
       case SchedulingMode.FIFO =>
         new FIFOSchedulingAlgorithm()
+      case _ =>
+        throw new IllegalArgumentException(s"Unsupported spark.scheduler.mode: $schedulingMode")
     }
   }
 

core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala

@@ -135,6 +135,8 @@ private[spark] class TaskSchedulerImpl(
           new FIFOSchedulableBuilder(rootPool)
         case SchedulingMode.FAIR =>
           new FairSchedulableBuilder(rootPool, conf)
+        case _ =>
+          throw new IllegalArgumentException(s"Unsupported spark.scheduler.mode: $schedulingMode")
       }
     }
     schedulableBuilder.buildPools()