Ensure that we always warn, with source, when we get an unparseable HsDesc or bridge desc.

There's a low-strength client fingerprinting attack where a bridge or onion service tries different variant descriptors to see which ones a given client is willing to parse, and thereby partitions clients by their software version.

As a mitigation (discussed in torspec#296 (closed)), we might want to ensure that we always warn noisily when we get an unparseable descriptor of this kind, and include information about the descriptor's source: this would at least make the attack less attractive.

I'm putting this in Icebox since it is far from the best attack available; the only reason that I'm opening a ticket at all is that the mitigation is so simple.