extract_appstream: debug if an exported appstream is skipped
extract_appstream: debug if an exported appstream is skipped

We had missing icons for a few apps due to this - if this ever happens
it means something very weird is going on with the export process in
a flatpak, so it's probably worth alerting the repo admin.

Closes: #1367
Approved by: pwithnall

flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak