Guard against Mesos cluster mode with R

Author: Andrew Or

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

@@ -322,6 +322,9 @@ object SparkSubmit {
       case (MESOS, CLUSTER) if args.isPython =>
         printErrorAndExit("Cluster deploy mode is currently not supported for python " +
           "applications on Mesos clusters.")
+      case (MESOS, CLUSTER) if args.isR =>
+        printErrorAndExit("Cluster deploy mode is currently not supported for R " +
+          "applications on Mesos clusters.")
       case (STANDALONE, CLUSTER) if args.isPython =>
         printErrorAndExit("Cluster deploy mode is currently not supported for python " +
           "applications on standalone clusters.")