[SPARK-10481] [YARN] SPARK_PREPEND_CLASSES make spark-yarn related jar could n…

Throw a more readable exception. Please help review. Thanks

Author: Jeff Zhang <zjffdu@apache.org>

Closes #8649 from zjffdu/SPARK-10481.

yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -1045,7 +1045,10 @@ object Client extends Logging {
           s"in favor of the $CONF_SPARK_JAR configuration variable.")
       System.getenv(ENV_SPARK_JAR)
     } else {
-      SparkContext.jarOfClass(this.getClass).head
+      SparkContext.jarOfClass(this.getClass).getOrElse(throw new SparkException("Could not "
+        + "find jar containing Spark classes. The jar can be defined using the "
+        + "spark.yarn.jar configuration option. If testing Spark, either set that option or "
+        + "make sure SPARK_PREPEND_CLASSES is not set."))
     }
   }