Add a warning

Author: zsxwing

core/src/main/scala/org/apache/spark/scheduler/ReplayListenerBus.scala

@@ -78,6 +78,7 @@ private[spark] class ReplayListenerBus extends SparkListenerBus with Logging {
             =>
             // Ignore events generated by Structured Streaming in Spark 2.0.0 and 2.0.1.
             // It's safe since no place uses them.
+            logWarning(s"Dropped incompatible Structured Streaming log: $currentLine")
           case jpe: JsonParseException =>
             // We can only ignore exception from last line of the file that might be truncated
             // the last entry may not be the very last line in the event log, but we treat it