Improve error messaging when repo directory is deleted

[gitbutler-client]

https://discord.com/channels/1060193121130000425/1206670506271707156/1210892156047458344

hazrpg wrote:

I've got a new bug for you - if the directory for a repo is deleted you get a generic error. Obviously the error is clear enough but its not user-friendly. It might be worth handling this.

This could be a discussion though, because: should it just say that the repo is missing/deleted/gone; Or should it handle it and remove it automatically but give you a heads up about it.