Logged out during editing or posting: intercept and report, don't fail silently

[cellio]

meta:291621

If you start to edit a post (and I assume also if you start to create one) and get logged out, then when you go to submit the edit it fails and then the button becomes disabled, but you don't get any sort of error saying what the problem is. We don't do live updates via websockets or the like, so the page you're on can't know you've been signed out when you click the button, but when we get the failure back from the server, it would be better if we popped up an error message saying you need to log in.

One thing to check: are we at risk of losing the draft? When drafts are saved, are they tied to your browser session (it'll still be there when you log in) or to the current user? Make sure we account for that when addressing this. A big complicated edit that took you long enough that you got logged out behind the scenes is exactly the type of edit we do not want to drop on the floor.