Handle non-200 requests

[johnholdun]

When features that involve fetching external documents receive unexpected responses, their behavior should change. Generally, this means that they should fall back to working like regular, unenhanced links and forms.

For example, if an inline link fetches a URL that returns a response with a status code in the 400 or 500 range, it should just display the full response as if the user clicked a regular link, as we cannot ensure that the targeted elements actually exist on the page or that the fetched content can reliably be rendered on the current page. An embed that returns a 400+ response should do nothing, leaving the enhanced link on the page for the user to click if they so desire.

300 requests should probably be followed, which is the default behavior when using fetch, so there's no work to be done here.