shared/install: report invalid unit files slightly better

shared/install: report invalid unit files slightly better

When a unit file is invalid, we'd return an error without any details:
$ systemctl --root=/ enable testing@instance.service
Failed to enable: Invalid argument.

Fix things to at least print the offending file name:
$ systemctl enable testing@instance.service
Failed to enable unit: File testing@instance.service: Invalid argument

$ systemctl --root=/ enable testing@instance.service
Failed to enable unit, file testing@instance.service: Invalid argument.

A real fix would be to pass back a proper error message from conf-parser.
But this would require major surgery, since conf-parser functions now
simply print log errors, but we would need to return them over the bus.
So let's just print the file name, to indicate where the error is.

(Incomplete) fix for #4210.