ucgi/: Return useful status codes when things go wrong.

It's simply wrong for the ucgi machinery to report status 200 OK when it
encounters trouble (e.g., the CGI program doesn't actually exist).
So report a useful status code as part of `error' or `syserror'.  We
assume that the latter is always a server-side error; the former might
be either, so we must annotate each call as appropriate.