[SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode

Added a simple checking for SparkContext.
Also added two rational checking against null at AM object.

Author: ehnalis <zoltan.zvara@gmail.com>

Closes #6083 from ehnalis/cluster and squashes the following commits:

926bd96 [ehnalis] Moved check to SparkContext.
7c89b6e [ehnalis] Remove false line.
ea2a5fe [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
4924e01 [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
39e4fa3 [ehnalis] SPARK-7504 [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
9f287c5 [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode

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

@@ -371,6 +371,14 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
       throw new SparkException("An application name must be set in your configuration")
     }
 
+    // System property spark.yarn.app.id must be set if user code ran by AM on a YARN cluster
+    // yarn-standalone is deprecated, but still supported
+    if ((master == "yarn-cluster" || master == "yarn-standalone") &&
+        !_conf.contains("spark.yarn.app.id")) {
+      throw new SparkException("Detected yarn-cluster mode, but isn't running on a cluster. " +
+        "Deployment to YARN is not supported directly by SparkContext. Please use spark-submit.")
+    }
+
     if (_conf.getBoolean("spark.logConf", false)) {
       logInfo("Spark configuration:\n" + _conf.toDebugString)
     }