datadir-apps: ignore ENOENT reading the .desktop filename

The desktopdb plugin can sometimes give us a non existing .desktop (for
example if the cache was not regenerated after uninstalling the
app), spare the warning in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=707728

Signed-off-by: Richard Hughes <richard@hughsie.com>