Bug: notify user in GUI if database is read-only

[aurelienpierre]

It can happen that restoring an old snapshot from library.db makes the file read-only, which silently makes picture selections fail but only shows a meaningful error when starting Ansel from CLI.

Edit: it can also deeply mess-up core features in a way that produces zero valid input to debug.

Need some GUI popup to notify, but that might require catching the exception from SQL bindings…