logind: more specific error message for unknown users

logind: more specific error message for unknown users

If you try to run `loginctl user-status` on a non-logged in user to see
whether "Linger" is enabled, it doesn't work.

If you're already an expert in logind, the fact that the user is considered
unknown actually tells you the user is not lingering.  So, probably they
they do not have lingering enabled.  I think we can point towards this
without being misleading.

I also reword it because I thought it was slightly confusing to run
`loginctl user-status root` and get an error back about "User 0".  Try to
be more specific, that it is "User ID 0".