shared/install: log syntax error for invalid DefaultInstance=

Ideally, we would want to report this over back over dbus. But that is pretty hard,
because the unitfile parsing logic doesn't provide any feedback.
systemd-analyze verify also doesn't notice the issue, because it doesn't look
at the [Install] section at all. Let's print a message in the logs at least.

(cherry picked from commit e5171296)
(cherry picked from commit 0757ad56)