repo: return error code if no successful mirrors

[abenson]

Should preserve the "its okay for a mirror to fail" behavior, but still catch in the case that it wasn't able to check any mirrors at all.

Closes #293.

[Duncaen]

As soon as you have one local repo, it wouldn't fail.

[ericonr]

Is that necessarily bad? If you have configured local repos you should know what you're about; in the case you don't have any local repos, failing to fetch from any repository means something is definitely diverting from expected.