Indicate if known (trusted) directory authories did not sign consensus
While adding the newest directory authority (longclaw) the consensus was not signed by enough trusted dirauths for clients to trust the document. It would be nice if consensus-health can generate a warning (e.g. make it obvious, flag) if we ever enter this state again where a consensus was reached/created but clients will not accept it. It'd also be useful to show which dirauths weren't included/which one(s) caused this failure.