[ONOS-3013]fix VTN bug: unify output of subnet rest api

Change-Id: I1b03010ff27b55bb8d2679f0e6d0945717f340d6
diff --git a/apps/vtnweb/src/main/java/org/onosproject/vtnweb/resources/SubnetWebResource.java b/apps/vtnweb/src/main/java/org/onosproject/vtnweb/resources/SubnetWebResource.java
index d886d92..deb9ca3 100644
--- a/apps/vtnweb/src/main/java/org/onosproject/vtnweb/resources/SubnetWebResource.java
+++ b/apps/vtnweb/src/main/java/org/onosproject/vtnweb/resources/SubnetWebResource.java
@@ -70,7 +70,7 @@
 public class SubnetWebResource extends AbstractWebResource {
     private final Logger log = LoggerFactory.getLogger(SubnetWebResource.class);
     public static final String SUBNET_NOT_CREATE = "Subnets is failed to create!";
-    public static final String SUBNET_NOT_FOUND = "Subnets is failed to update!";
+    public static final String SUBNET_NOT_FOUND = "Subnets is not found";
     public static final String JSON_NOT_NULL = "JsonNode can not be null";
 
     @GET