admin > settings > directories should validate directories before saving bad values into the db

If the user enters a path that doesn't exist and can't be created due to permissioning issues, CiviCRM is borked and the user must set civicrm_domain.config_backend to NULL. This happens to me all the time when I get confused about which server I'm on and where the webroot is.

What should happen here is that CiviCRM should check the form values on the validation stage and flag errors before saving faulty values to the db.