Chat: cannot wait for changes in a file if file for today hasn't been created yet

[timbl]

The long chat system only creates a file for chat in a given day if someone actually sends a message. After it is created, each user of the chat subscribes to web socket events when the file changes.
Anyone opening the chat before the file for today has been created gets an error because it hasn't got a 200 response from the file and so can't look at the headers in that.

But in fact the 404 response does have a Updates-Via: wss://example.com header.

So it needs the rdflib function which looks at the headers at it to also look at 404 responses. And it needs the server to allow one to watch for a non existent file being created.