Explicit responses should bypass error pages

Currently if `text/html` is accepted, Up will serve the error page. It seems common for people to `fetch()` or `curl` without explicitly setting `Accept: application/json` or similar, resulting in the HTML which can be confusing.

We could disable error pages by default, or maybe drop content negotiation and assume non-text/plain responses are intentional, something like that.

Note to self: at least document Accept usage