Cached directory object from the future makes us fail bootstrapping; is this right?

Our current behavior, when starting with a consensus from the future, is to fail to bootstrap, and exit the bootstrap function with an error: since the consensus is from the future and validated, our time is necessarily in the past, and bootstrapping cannot succeed.

Several things to do here:

• Make the error message better.
• Make sure that we only do this for a consensus that we have authenticated as well-signed by the authorities we believe in.
• Think about whether this is truly what we want.