document portal: Make error message more helpful
document portal: Make error message more helpful

When we fail to load the database, point out the file.

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal