app/flatpak-builtins-update: skip refs from unreachable remotes
app/flatpak-builtins-update: skip refs from unreachable remotes

Allow refs to be added to the update operation in "flatpak update" even if some
of the matching group have a missing remote. In the case this leaves the
transaction empty, the "nothing to do" error will be printed after the
transaction is run.

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak