prop224: Dump a malformed descriptor in a file and log_warn about it
Once a client fetches a descriptor, it could be unparseable thus malformed. Dump it in a file and warn about it so the user can report it properly to us and we also don't have a 50kb blob of text in the logs.
Worth thinking if we might want that only with
SafeLogging 0 since leaving HS descriptors on disk client side might not be ideal?
As for v2 subsystem, the onion address is in the descriptor so I would be very careful to put that on disk.