missing permission to create config files

In my Hosting www-data hasn´t the permissions to create folders/files in the plugin directory.
When I try to save any settings there is No error message. And the changes simply disappear.

It was hard to find out why I`m not able to config your plugin.

So please add an error message, wehen it`s not possible to create/change the config.
While this isn`t implementet add this problem to your faq.

Greets

http://wordpress.org/plugins/ad-injection/