dev server 404 redirect should show 404 page

[jaeh]

there will always be a 404 page.

currently, the dev server router will redirect to WEB_ROOT on 404.

that logic should instead only redirect to WEB_ROOT if the reason for the 404 is that the WEB_ROOT is missing from the start of the request.url,
serving the 404.html page on all other 404s.