sysusers: handle NSS errors gracefully

sysusers: handle NSS errors gracefully

If the io.systemd.DynamicUser or io.systemd.Machine files exist,
but nothing is listening on them, the nss-systemd module returns
ECONNREFUSED and systemd-sysusers fails to creat the user/group.

This is problematic when ran by packaging scripts, as the package
assumes that after this has run, the user/group exist and can
be used. adduser does not fail in the same situation.

Change sysusers to print a loud warning but otherwise continue
when NSS returns an error.