In SvelteKit, users can return a Promise from a load function which causes a streamed http respnse to be sent. If the promise rejects after it is returned and the initial response is sent, an error is thrown server-side. This error will not be captured by our request handler wrappers because at this time, they already finished.

However, the error will be caught via our handleErrorWithSentry hook. This means we actually still catch the error but because the cloudflare function already terminated directly after the rejection, it is not sent successfully (i.e. the good old flush problem).

This PR makes sure we use of Cloudflare's waitUntil function which fortunately is available within the error handler hook.

closes #17009