This adds a new error to FlatpakError and uses it everywhere a ref is not
found, either locally or in a remote. This should hopefully be more useful than
the status quo of either returning FLATPAK_ERROR_INVALID_DATA or
G_IO_ERROR_NOT_FOUND or something else. Technically this is an API break but it
seems worth the risk. I checked gnome-software which does not seem affected by
this, and I checked eos-updater which does check for G_IO_ERROR_NOT_FOUND in
one place that will be affected by this but we can patch that.
Closes: #2895
Approved by: matthiasclasen
c0d5f1bfc8