[feat] allow to render error pages from handleError

[nhe23]

Fixes #2824 by optionally rendering error pages if renderError is returned from the handleError hook. This solution is better than #3171 because the right status code is returned and the user can reload the page in order to try again.

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[Rich-Harris]

Thank you. Adding new API is unnecessary though — all we need to do is this...

// pseudo-code
try {
  return handle();
} catch (e) {
  handle_error(e);
  return render_error_page();
}

...and wrap all the error page rendering logic in a try-catch (previously, some of it was outside the try-catch) so that that's guaranteed not to throw. I've opened #3239, so I'll close this and #3171