[SPARK-4787] Stop SparkContext if a DAGScheduler init error occurs

Author: Dale <tigerquoll@outlook.com>

Closes apache#3809 from tigerquoll/SPARK-4787 and squashes the following commits:

5661e01 [Dale] [SPARK-4787] Ensure that call to stop() doesn't lose the exception by using a finally block.
2172578 [Dale] [SPARK-4787] Stop context properly if an exception occurs during DAGScheduler initialization.

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -329,8 +329,13 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   try {
     dagScheduler = new DAGScheduler(this)
   } catch {
-    case e: Exception => throw
-      new SparkException("DAGScheduler cannot be initialized due to %s".format(e.getMessage))
+    case e: Exception => {
+      try {
+        stop()
+      } finally {
+        throw new SparkException("Error while constructing DAGScheduler", e)
+      }
+    }
   }
 
   // start TaskScheduler after taskScheduler sets DAGScheduler reference in DAGScheduler's