extensionUtils: don't log verbosely on missing extension directory

A missing extension directory isn't worth debug spew, so check
if the error when reading the extension directory is NOT_FOUND,
and if so, suppress output.

https://bugzilla.gnome.org/show_bug.cgi?id=685466

js/misc/extensionUtils.js

@@ -161,6 +161,7 @@ const ExtensionFinder = new Lang.Class({
         try {
             fileEnum = dir.enumerate_children('standard::*', Gio.FileQueryInfoFlags.NONE, null);
         } catch(e) {
+            if (e.domain != Gio.io_error_quark() || e.code != Gio.IOErrorEnum.NOT_FOUND)
                 logError(e, 'Could not enumerate extensions directory');
             return;
         }