[MINOR] [CORE] Warn users who try to cache RDDs with dynamic allocation on.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5751 from vanzin/cached-rdd-warning and squashes the following commits:

554cc07 [Marcelo Vanzin] Change message.
9efb9da [Marcelo Vanzin] [minor] [core] Warn users who try to cache RDDs with dynamic allocation on.

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

@@ -1396,6 +1396,11 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    * Register an RDD to be persisted in memory and/or disk storage
    */
   private[spark] def persistRDD(rdd: RDD[_]) {
+    _executorAllocationManager.foreach { _ =>
+      logWarning(
+        s"Dynamic allocation currently does not support cached RDDs. Cached data for RDD " +
+        s"${rdd.id} will be lost when executors are removed.")
+    }
     persistentRdds(rdd.id) = rdd
   }