String formatting: Fix warning

Fix to not trigger a warning like:

JS ERROR: !!!   WARNING: 'anonymous function does not always return a value'

by adding a return ""; at the end of the anonymous function.

https://bugzilla.gnome.org/show_bug.cgi?id=595661

js/misc/format.js

@@ -39,6 +39,6 @@ function format() {
                         default:
                             throw new Error('Unsupported conversion character %' + genericGroup);
                     }
-
+                    return ""; // Suppress warning
                 });
 }