respect Accept header

[jbearer]

• currently we return html error messages in certain cases (eg url doesn't match any route) this is nice for experimentation but it makes automated clients harder to debug, as they will often discard the response body if it can't be parsed as json
• we should always return a json object with the same shape when there is any error and the Accept header asks for json
• in the browser, Accept will be unset or set to html, and we can still return friendly html