build_downloader: ignore things that aren't build directories, and show a more useful error when the tree has been closed for a few days
build_downloader: ignore things that aren't build directories, and show a more useful error when the tree has been closed for a few days

nth10sd/funfuzz,nth10sd/funfuzz,MozillaSecurity/funfuzz,MozillaSecurity/funfuzz,nth10sd/funfuzz,MozillaSecurity/funfuzz