[SPARK-5698] Do not let user request negative # of executors

Otherwise we might crash the ApplicationMaster. Why? Please see https://issues.apache.org/jira/browse/SPARK-5698.

sryza I believe this is also relevant in your patch #4168.

Author: Andrew Or <andrew@databricks.com>

Closes #4483 from andrewor14/da-negative and squashes the following commits:

53ed955 [Andrew Or] Throw IllegalArgumentException instead
0e89fd5 [Andrew Or] Check against negative requests

core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala

@@ -314,6 +314,11 @@ class CoarseGrainedSchedulerBackend(scheduler: TaskSchedulerImpl, val actorSyste
    * Return whether the request is acknowledged.
    */
   final override def requestExecutors(numAdditionalExecutors: Int): Boolean = synchronized {
+    if (numAdditionalExecutors < 0) {
+      throw new IllegalArgumentException(
+        "Attempted to request a negative number of additional executor(s) " +
+        s"$numAdditionalExecutors from the cluster manager. Please specify a positive number!")
+    }
     logInfo(s"Requesting $numAdditionalExecutors additional executor(s) from the cluster manager")
     logDebug(s"Number of pending executors is now $numPendingExecutors")
     numPendingExecutors += numAdditionalExecutors