[fix] load errorTemplate from correct location

[dummdidumm]

The alternative would be to move the "set a fallback error.html path if not set" logic into load_error_page, which @Rich-Harris added here

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[Rich-Harris]

I think the other option is preferable — vite/dev/index.js is already an unwieldy monster, so anything to keep logic out of there is a good thing, and the less we need to restart the server the better