ONOS-944: Implemented a Group operation failure handler in GroupManger and SimpleGroupStore.

Change-Id: Ib3be4d534ceff04af2dad0c062fd4cd63d49ee82
diff --git a/core/api/src/main/java/org/onosproject/net/group/GroupEvent.java b/core/api/src/main/java/org/onosproject/net/group/GroupEvent.java
index d9d8a97..45fbb3e 100644
--- a/core/api/src/main/java/org/onosproject/net/group/GroupEvent.java
+++ b/core/api/src/main/java/org/onosproject/net/group/GroupEvent.java
@@ -41,6 +41,21 @@
          */
         GROUP_UPDATED,
 
+        /**
+         * Signifies that a request to create Group has failed.
+         */
+        GROUP_ADD_FAILED,
+
+        /**
+         * Signifies that a request to remove Group has failed.
+         */
+        GROUP_REMOVE_FAILED,
+
+        /**
+         * Signifies that a request to update Group has failed.
+         */
+        GROUP_UPDATE_FAILED,
+
         // internal event between Manager <-> Store
 
         /*
@@ -55,6 +70,8 @@
          * Signifies that a request to delete Group has been added to the store.
          */
         GROUP_REMOVE_REQUESTED,
+
+
     }
 
     /**