Getting 404 for a mirror URL should give a useful error message
Currently:
[15:14:28] Download https://build.postmarketos.org/wip/master/master/aarch64/APKINDEX.tar.gz
[15:14:28] WARNING: file not found: https://build.postmarketos.org/wip/master/master/aarch64/APKINDEX.tar.gz
[15:14:28] ERROR: expected str, bytes or os.PathLike object, not NoneType