logind: give better error messages when failing to attach devices to seats
When the user tries to attach a device lacking ID_FOR_SEAT they
currently get a very cryptic error message. Let's improve the situation
a bit. Still a bit cryptic maybe, but much less so.
Inspired-by: https://lists.freedesktop.org/archives/systemd-devel/2023-September/049469.html
Inspired-by: https://lists.freedesktop.org/archives/systemd-devel/2023-September/049484.html
Also-see: https://lists.freedesktop.org/archives/systemd-devel/2023-September/049470.html
Also-see: https://lists.freedesktop.org/archives/systemd-devel/2023-September/049489.html