Turbopack: Use `SharedError` for body streaming (#47677)
Pending https://github.com/vercel/turbo/pull/4392 landing in Turbopack (and https://github.com/vercel/next.js/pull/47476 landing here), this removes `BodyError` and switches to `SharedError`. That should allow us to preserve the source chain of errors for when we finally display it to the dev, aiding debugging.
