systemctl: clearer error message for missing install information

systemctl: clearer error message for missing install information

Some users found it difficult to understand what systemctl was telling
them.
Instead of "install information" talk about "[Install] section", which
is more likely to ring a bell. And suggest that it is intentional, so
that users do not attempt to "correct" the unit files.

https://bugzilla.redhat.com/show_bug.cgi?id=817033