fix: don't crawl error responses during prerendering

[Rich-Harris]

closes #14542. No test because we would have to create a whole new test app, and this is a real edge case (you have to have a relative link with multiple segments on the root layout or error page and you have to have neutered handleHttpError which you should almost never do)

this diff vs #14565 is why i'm not worried about the robots taking our jobs yet

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