Improve use of http.Error. github.com/shurcooL/issuesapp#20

Calling it after the header has been written (likely with the wrong content-type) is incorrect and shouldn't be done.

A good pattern I've since found to take care of this is to have a func(w http.ResponseWriter, req *http.Request) error handler signature, and keep track of whether the response writer has been written to. See here.

Resolved a while ago in commit 431bb22d.