Improve error log message

Change-Id: I87dcb733155675591c95bd9a2001c1be9ff1b392
diff --git a/runtime/src/main/java/org/onosproject/yang/runtime/impl/DefaultYangModelRegistry.java b/runtime/src/main/java/org/onosproject/yang/runtime/impl/DefaultYangModelRegistry.java
index 5d7c0e6..a586167 100644
--- a/runtime/src/main/java/org/onosproject/yang/runtime/impl/DefaultYangModelRegistry.java
+++ b/runtime/src/main/java/org/onosproject/yang/runtime/impl/DefaultYangModelRegistry.java
@@ -354,9 +354,9 @@
             if (regClass == null) {
                 regClass = registerClassStore.get(interfaceName);
             }
-        }
-        if (regClass == null) {
-            log.error("{} node should not be null.");
+            if (regClass == null) {
+                log.error("Nothing registered for {} or {}", serviceName, interfaceName);
+            }
         }
         return regClass;
     }