SPARK-3014. Log a more informative messages in a couple failure scenario...

...s

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1934 from sryza/sandy-spark-3014 and squashes the following commits:

ae19cc1 [Sandy Ryza] SPARK-3014. Log a more informative messages in a couple failure scenarios

core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala

@@ -18,7 +18,7 @@
 package org.apache.spark.deploy
 
 import java.io.{File, PrintStream}
-import java.lang.reflect.InvocationTargetException
+import java.lang.reflect.{Modifier, InvocationTargetException}
 import java.net.URL
 
 import scala.collection.mutable.{ArrayBuffer, HashMap, Map}
@@ -323,7 +323,9 @@ object SparkSubmit {
     }
 
     val mainMethod = mainClass.getMethod("main", new Array[String](0).getClass)
-
+    if (!Modifier.isStatic(mainMethod.getModifiers)) {
+      throw new IllegalStateException("The main method in the given main class must be static")
+    }
     try {
       mainMethod.invoke(null, childArgs.toArray)
     } catch {

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -283,11 +283,9 @@ private[spark] class ApplicationMaster(args: ApplicationMasterArguments,
         }
 
         val sparkContext = sparkContextRef.get()
-        assert(sparkContext != null || count >= numTries)
         if (sparkContext == null) {
-          logError(
-            "Unable to retrieve sparkContext inspite of waiting for %d, numTries = %d".format(
-              count * waitTime, numTries))
+          logError(("SparkContext did not initialize after waiting for %d ms. Please check earlier"
+            + " log output for errors. Failing the application.").format(numTries * waitTime))
         }
         sparkContext
       }