install: warn if WantedBy targets don't exist

Currently, if [Install] section contains WantedBy=target that doesn't exist,
systemd creates the symlinks anyway. That is just user-unfriendly.
Let's be nice and warn about installing non-existent targets.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1835351.

Replaces: #15834