ONOS-4721 fix

Change-Id: Idb0ba949293451488b350c7cdcfc50094a3c9abc

@@ -146,8 +146,7 @@ public final class GroupCodec extends JsonCodec<Group> {
                 groupType = Group.Type.FAILOVER;
                 break;
             default:
-                log.warn("The requested type {} is not defined for group.", type);
-                return null;
+                nullIsIllegal(groupType, "The requested group type " + type + " is not valid");
         }
 
         // parse group buckets