Improve external storage configuration UI

This PR will track several changes to the external storages UI. When complete, fixes #7737, fixes #7738.

First change is the highlighting of fields that are not optional and are not filled.

Fixes #10511, fixes #8174 
No, this is necessary for a proper exception hierarchy. If I want to catch all files_external exceptions, I can just catch this exception, which all other exceptions in the app are derived from.