Privatize error messages.

Change-Id: Id6eb4119f677141cb827431effa428ecbd57a530

@@ -65,12 +65,12 @@ public class TopologyManager
         TopologyProvider, TopologyProviderService>
         implements TopologyService, TopologyProviderRegistry {
 
-    public static final String TOPOLOGY_NULL = "Topology cannot be null";
+    private static final String TOPOLOGY_NULL = "Topology cannot be null";
     private static final String DEVICE_ID_NULL = "Device ID cannot be null";
     private static final String CLUSTER_ID_NULL = "Cluster ID cannot be null";
     private static final String CLUSTER_NULL = "Topology cluster cannot be null";
-    public static final String CONNECTION_POINT_NULL = "Connection point cannot be null";
-    public static final String LINK_WEIGHT_NULL = "Link weight cannot be null";
+    private static final String CONNECTION_POINT_NULL = "Connection point cannot be null";
+    private static final String LINK_WEIGHT_NULL = "Link weight cannot be null";
 
     private final Logger log = getLogger(getClass());