Client: Fix forward-validation failure messages

Forward validation failures were never updated to use the new Failure
object, so they ended up showing the default message.  The problem was that
`ui.invalidateForm' was replacing the Failure object with a new one with an
empty message.

This change just does the same thing as the change and save events.

* src/client/Client.js (_forwardValidate): Invoke `#_genValidationMessages'
    and `_dataValidator.updateFailures'.  Stop calling `ui.invalidateForm'.

src/client/Client.js

@@ -2046,7 +2046,13 @@ module.exports = Class( 'Client' )
         // just as we would with the `submit' event.
         if ( failures !== null )
         {
-            this.ui.invalidateForm( failures );
+            this._genValidationMessages(
+                this._validationMessages,
+                failures
+            );
+
+            this._dataValidator.updateFailures( {}, failures );
+
             return false;
         }