* Remove d2config.Config singleton
* refactored config file bootstrap
* `d2loader.Loader` adds the config directories during init
* `d2asset.AssetManager` loads the config file during init
* mpq verification logic removed from d2config; this is done by d2loader
* added `errorMessage` to `d2app.App` for setting the error message for the error screen.
* fixed loader test
