[SPARK-2312] Logging Unhandled messages

The  previous solution has changed based on https://github.com/apache/spark/pull/2048 discussions.

Author: Isaias Barroso <isaias.barroso@gmail.com>

Closes #2055 from isaias/SPARK-2312 and squashes the following commits:

f61d9e6 [Isaias Barroso] Change Log level for unhandled message to debug
f341777 [Isaias Barroso] [SPARK-2312] Logging Unhandled messages

core/src/main/scala/org/apache/spark/util/ActorLogReceive.scala

@@ -43,7 +43,13 @@ private[spark] trait ActorLogReceive {
 
     private val _receiveWithLogging = receiveWithLogging
 
-    override def isDefinedAt(o: Any): Boolean = _receiveWithLogging.isDefinedAt(o)
+    override def isDefinedAt(o: Any): Boolean = {
+      val handled = _receiveWithLogging.isDefinedAt(o)
+      if (!handled) {
+        log.debug(s"Received unexpected actor system event: $o")
+      }
+      handled
+    }
 
     override def apply(o: Any): Unit = {
       if (log.isDebugEnabled) {