Added the error information to the toString() if

there is error information to show.

jme3-networking/src/main/java/com/jme3/network/service/rpc/msg/RpcResponseMessage.java

@@ -84,6 +84,7 @@ public class RpcResponseMessage extends AbstractMessage {
     @Override
     public String toString() {
         return getClass().getSimpleName() + "[#" + msgId + ", result=" + result
+                                          + (error != null ? ", error=" + error : "")
                                           + "]";
     }
 }