get_koji_images(): log a warning when no Koji images found
    
    It's possible that for a nightly *all* Koji tasks might fail,
    but we still have non-Koji images (boot.iso). Before the multi-
    processing / multicall / koji_done changes, we were catching
    in _check_koji_urls(), which has gone away now. get_koji_
    images() seems like the best place to do it now. Let's log a
    warning.