🐛 Bug Report — `ReadableStream` don't propagate into `fetch` requests

See for reproduction: https://github.com/cloudflare/workerd/pull/4543

Opening an issue mainly for visibility, since it's unclear if this was acknowledged as a bug.

**Note:** More details can be found on the reproduction than in this issue.

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When a `ReadableStream` fails (or a converted `AsyncIterable`) any error on the source isn't observable on the `fetch` request and this doesn't propagate into a cancellation of `BodyStream` or the `fetch` request itself.

The error becomes unobservable and unhandled.

Workarounds here are problematic, because they require a conscious check of whether a `ReadableStream` enters any `fetch` request. This also makes this hard to verify. Any workaround with an `AbortController` doesn't guarantee that the error will propagate and an unhandled rejection will still occur.