fixes https://linear.app/tryghost/issue/ENG-746/http-500-responses-when-handle-image-sizes-middleware-hits-missing
- in the event a request comes in for a resized image, but the source
image does not exist, we return a rendered 404 page
- we do this because we pass the NotFoundError to `next`, which skips
over the static asset code where we return a plaintext 404
- also included a breaking test that ensure we go to the next middleware
without an error
