[SPARK-11786][CORE] Tone down messages from akka error monitor.

There events happen normally during the app's lifecycle, so printing
out ERROR logs all the time is misleading, and can actually affect usability
of interactive shells.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9772 from vanzin/SPARK-11786.

core/src/main/scala/org/apache/spark/rpc/akka/AkkaRpcEnv.scala

@@ -263,7 +263,7 @@ private[akka] class ErrorMonitor extends Actor with ActorLogReceive with Logging
   }
 
   override def receiveWithLogging: Actor.Receive = {
-    case Error(cause: Throwable, _, _, message: String) => logError(message, cause)
+    case Error(cause: Throwable, _, _, message: String) => logDebug(message, cause)
   }
 }