smb: keep stderr of net commands on add/remove
this allows guis to show the backend stderr on UserShareSystemError when we haven't managed to map that to a more specific type (which we currently do not ever; there's an unresolved todo about this).
this allows guis to show the backend stderr on UserShareSystemError when we haven't managed to map that to a more specific type (which we currently do not ever; there's an unresolved todo about this).