More debug messages

external/flume/src/main/scala/org/apache/spark/streaming/flume/FlumePollingInputDStream.scala

@@ -116,15 +116,17 @@ private[streaming] class FlumePollingReceiver(
                     logDebug("Stored events with seq:" + seq)
                     j += 1
                   }
-                  logDebug("Sending ack for: " +seq)
+                  logDebug("Sending ack for sequence number: " +seq)
                   // Send an ack to Flume so that Flume discards the events from its channels.
                   client.ack(seq)
                   logDebug("Ack sent for sequence number: " + seq)
                 } catch {
                   case e: Exception =>
                     try {
                       // Let Flume know that the events need to be pushed back into the channel.
+                      logDebug("Sending nack for sequence number: " + seq)
                       client.nack(seq) // If the agent is down, even this could fail and throw
+                      logDebug("Nack sent for sequence number: " + seq)
                     } catch {
                       case e: Exception => logError(
                         "Sending Nack also failed. A Flume agent is down.")