[MINOR][DSTREAMS] Add DStreamCheckpointData.cleanup warning if delete returns false

## What changes were proposed in this pull request?

While I was reviewing apache#24235 I've found a minor addition possibility. Namely `FileSystem.delete` returns a boolean which is not yet checked. In this PR I've added a warning message when it returns false. I've added this as MINOR because no control flow change introduced.

## How was this patch tested?

Existing unit tests.

Closes apache#24263 from gaborgsomogyi/SPARK-27301-minor.

Authored-by: Gabor Somogyi <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: gaborgsomogyi

streaming/src/main/scala/org/apache/spark/streaming/dstream/DStreamCheckpointData.scala

@@ -87,9 +87,12 @@ class DStreamCheckpointData[T: ClassTag](dstream: DStream[T])
               if (fileSystem == null) {
                 fileSystem = path.getFileSystem(dstream.ssc.sparkContext.hadoopConfiguration)
               }
-              fileSystem.delete(path, true)
+              if (fileSystem.delete(path, true)) {
+                logInfo("Deleted checkpoint file '" + file + "' for time " + time)
+              } else {
+                logWarning(s"Error deleting old checkpoint file '$file' for time $time")
+              }
               timeToCheckpointFile -= time
-              logInfo("Deleted checkpoint file '" + file + "' for time " + time)
             } catch {
               case e: Exception =>
                 logWarning("Error deleting old checkpoint file '" + file + "' for time " + time, e)