[ONOS-2499]update accordingly for ONOS-2408's review recommendation

Change-Id: I24e7c4acbe38e4286b994c5bf50d22def945fb29
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/Condition.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/Condition.java
index 40aab68..cbf3542 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/Condition.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/notation/Condition.java
@@ -64,9 +64,9 @@
      * @param value column data
      */
     public Condition(String column, Function function, Object value) {
-        checkNotNull(column, "column is not null");
-        checkNotNull(function, "function is not null");
-        checkNotNull(value, "value is not null");
+        checkNotNull(column, "column cannot be null");
+        checkNotNull(function, "function cannot be null");
+        checkNotNull(value, "value cannot be null");
         this.column = column;
         this.function = function;
         this.value = value;