The "Discard Draft" action would fail silently if something went wrong (for

example, if the server was inaccessible or the user was no longer logged in).

This change does some trivial plumbing to show the user an error in this case

(in the same way that we show other highly unexpected errors).

Loaded the page, killed my devserver, and tried to discard a draft. Saw an

alert that said "HTTP 0 error", which is at least better than nothing.