[mirror] allow 404 for indices.
[mirror] allow 404 for indices.

Some repositories including the official ubuntu archive sometimes
have Release/InRelease files that list non-existent indices.

Specifically, I found http://archive.ubuntu.com/ubuntu/dists/trusty-updates/Release
lists multiverse/binary-amd64/Release which does not exist.

This commit allows such missing indices.

cybozu-go/go-apt-cacher,cybozu-go/aptutil,cybozu-go/aptutil