Invalid JSON messages triggers an unhandled exception in JsonWebsocketConsumer and AsyncJsonWebsocketConsumer

[JulienPalard]

Hi!

On malformed json sent by clients to JsonWebsocketConsumer or AsyncJsonWebsocketConsumer, a JSONDecodeError is raised and left unhandled.

The unhandled exception stops the django process (which will be restarted by Daphne so that's OKish).

But this looks fragile to me: a malicious user could spam malformed messages, with various effects like having the admin inbox to quickly fill if AdminEmailHandler is used.

I think malformed messages should be logged and ignored, but not raise an unhandled exception, what do you think?

Related to #1339

[carltongibson]

Hey @JulienPalard, I'd imagine here that you've want to send some kind of error response to the client. (Something equivalent to a 400 Bad Request in HTTP land.)

I guess first thing is to show the customisations there catching the error from or in decode_json().

(It's not immediately clear that we'd ship that as part of Channel's basic consumers, but let's see what it looks like.)