build-bundle: Improve error handling
build-bundle: Improve error handling

Don't fail in obscure ways if commandline options don't
look like files.

Closes: #2191

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