handle non-200 status codes from root __layout.svelte

[Rich-Harris]

#4659. Just a failing test for now. Looks like the 'bail out and render a generic 500 if an error occurs while rendering an error page' thing only applies to errors that are thrown; return { status: 404 } doesn't do the trick.

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[Conduitry]

I haven't fully been following what's happening in that issue - but from your description in this PR, it sounds at least somewhat adjacent to #3068 - although the symptoms are different