Resolve a KDE4 TODO in KDesktopFileActions

Show an error message when KDesktopFileActions::run() is called with a
desktop file that looks like it's an application or runnable service,
but KService actually doesn't like it.

The comment about the error message already having been shown hasn't
been true for around 15 years, as far as I can tell (git commit
7bda6331, svn revision 20718).

REVIEW: 117299