extensionSystem: Fix an error related to extension importing

If an extension fails to import, we will pass the error object
to logExtensionError, which fails to pass it onto DBus as an
error object is not a string. To fix, convert the error object
to a string before passing it to logExtensionError.

https://bugzilla.gnome.org/show_bug.cgi?id=668429

js/ui/extensionSystem.js

@@ -394,7 +394,7 @@ function loadExtension(dir, type, enabled) {
     } catch (e) {
         if (stylesheetPath != null)
             theme.unload_stylesheet(stylesheetPath);
-        logExtensionError(uuid, e);
+        logExtensionError(uuid, '' + e);
         return;
     }