Added back-pipe for role assertion issues.
diff --git a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
index 2f61925..9b6c83a 100644
--- a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
@@ -224,6 +224,11 @@
                 post(event);
             }
         }
+
+        @Override
+        public void unableToAssertRole(DeviceId deviceId, MastershipRole role) {
+            // FIXME: implement response to this notification
+        }
     }
 
     // Posts the specified event to the local event dispatcher.