Clients should warn and disable guards responsible for excessive circuit failures

In https://lists.torproject.org/pipermail/tor-dev/2012-March/003361.html, the raccoon gave us some bounds to determine unreasonable amounts of circuit failure that indicate a potential tagging attack.

Tor Clients should emit warnings and disable guards if they observe anywhere near this amount of circuit failure per guard. His bound for tagging was 80%, but frankly if even 50% of my circuits are failing, I would want tor to tell me.

Maybe >50% failure should notice and >66% failure should warn?