[SPARK-4379][Core] Change Exception to SparkException in checkpoint

It's better to change to SparkException. However, it's a breaking change since it will change the exception type.

Author: zsxwing <zsxwing@gmail.com>

Closes #3241 from zsxwing/SPARK-4379 and squashes the following commits:

409f3af [zsxwing] Change Exception to SparkException in checkpoint

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -1202,7 +1202,7 @@ abstract class RDD[T: ClassTag](
    */
   def checkpoint() {
     if (context.checkpointDir.isEmpty) {
-      throw new Exception("Checkpoint directory has not been set in the SparkContext")
+      throw new SparkException("Checkpoint directory has not been set in the SparkContext")
     } else if (checkpointData.isEmpty) {
       checkpointData = Some(new RDDCheckpointData(this))
       checkpointData.get.markForCheckpoint()