Draft: imprv: Settings: Catch database exceptions and display error dialog

Catch database exceptions which could potentially be thrown while accessing the settings database.

Exceptions in the settings.py code caught by the decorator trigger an error Window which is modal, on top, an leaves no other options than to quit Gajim.

Part of #11555

Can be easily tested by starting Gajim, then renaming settings.db.

This window could offer users further actions, such as "Reset Database". I implemented this action as an example for errors with cache.db, but I did not add decorators there yet.