create stored_saving_location if not existing, fallback to saving_location on error

[VeldoraTheDragon]

Currently, the default directory Kooha gets created, if it's not existing.

However if a different directory is being set via dbus-option e.g saving-location=b'/home/{{ USERNAME }}/Screencasts' in corporate organizations via config-management, the directory doesn't get created automatically, instead it fails in an error.

This PR fixes this issue, by moving:

        let saving_location =
            glib::user_special_dir(glib::UserDirectory::Videos).unwrap_or_else(glib::home_dir);

further up and expanding !stored_saving_location.as.os.str().is_empty() with.

            if let Err(err) = fs::create_dir_all(&stored_saving_location) {
                tracing::warn!(
                    "Failed to create dir at `{}`: {:?}",
                    stored_saving_location.display(),
                    err
                );
                return saving_location;
            }

where a fallback to saving_location is set, in case it fails to create the directory set via saving-location option.

I've tested it on an Arch Linux machine by building it via a modified PKGBUILD of the official PKGBUILD and it worked flawlessly in my case.