systemctl: if service manager couldn't load unit file, don't rely on it to tell us...

systemctl: if service manager couldn't load unit file, don't rely on it to tell us the fragment path

Previously, "systemctl edit" exclusively used the service manager's
per-unit FragmentPath property to figure out which file to edit, when
operating on a non-template unit. If for some reason loading the unit
file failed entirely though (LoadState=error), then FragmentPath would
be empty, and thus the unit not editable.

Let's fix this, by falling back to client-side unit file searching in
this case.

(Also, various other clean-ups to make the relevant functions follow our
coding style)

Fixes: #9561