If DADEM is down, queue fills up with frozen messages

[abibroom]

unexpected error (type RABX::Error::Transport) while processing message: HTTP error for http://services.mysociety.org/dadem: 500 Status read failed: Connection reset by peer

froze message following error

They can be thawed manually via the admin, but ISTR we've had a problem before where there were so many frozen messages that the queue page simply wouldn't load, making that impossible.