shared/install: add some more debug messages and comments

shared/install: add some more debug messages and comments

$ systemctl --root=/ preset foobar.service
Cannot find unit foobar.service.
Failed to preset: No such file or directory.
$ systemctl --root=/ preset foobar@.service
Cannot find unit foobar@.service.
Failed to preset: No such file or directory.
$ systemctl --root=/ preset foobar@blah.service
Cannot find unit foobar@blah.service or foobar@.service.
Failed to preset: No such file or directory.