Show toast when "Set as wallpaper" portal fails, or hide menu item if portal unavailable
!808 (merged) removed the old wallpaper setting code, and after that setting the wallpaper is only possible when the desktop portal is installed. However, even if the desktop portal is not installed / not running, the "Set as wallpaper" option is still shown, but it does nothing, no error messages are shown on the user interface, only in the logs:
** (org.gnome.Nautilus:1354881): WARNING **: 15:07:41.850: Failed to set wallpaper via portal: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.portal.Desktop was not provided by any .service files