Warn instead of throw

... in case the user provides a NFS mounted directory.

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

@@ -1898,10 +1898,13 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    */
   def setCheckpointDir(directory: String) {
 
-    // If we are running on a cluster, require the path to be non-local
+    // If we are running on a cluster, log a warning if the directory is local.
+    // Otherwise, the driver may attempt to reconstruct the checkpointed RDD from
+    // its own local file system, which is incorrect because the checkpoint files
+    // are actually on the executor machines.
     if (!isLocal && Utils.nonLocalPaths(directory).isEmpty) {
-      throw new IllegalArgumentException(
-        "Checkpoint directory must be non-local if Spark is running on a cluster: " + directory)
+      logWarning("Checkpoint directory must be non-local " +
+        "if Spark is running on a cluster: " + directory)
     }
 
     checkpointDir = Option(directory).map { dir =>