Hide Let's Encrypt traceback, and replace it with a simpler message (when feasible)

The traceback provided by acme-tiny on error is terrifyingly ugly. It's informative sometimes, if you know how to read it, but for most users, their eyes glaze over and they don't read it (or if they do, don't understand it, which is reasonable in a lot of cases because it's not always clear without knowing how it all works what went wrong) and just keep clicking hoping it'll work.

Ideally, we'd catch the most common two or three error types (.well-known is unreachable, DNS not resolving, redirects maybe) by parsing the output and then punt on the ones that we aren't confident of and offer advice on what might be wrong. In all cases, it should be possible to click to see the traceback, of course, since we probably need to see it in support tickets.