[fiber] bugfix - don't show <Offscreen> in error message. (#35763)

## Overview

While building the RSC sandboxes I notice error messages like:

>  An error occurred in the `<Offscreen>` component

This is an internal component so it should show either:

>  An error occurred in the `<Suspense>` component.

>  An error occurred in the `<Activity>` component.

It should only happen when there's a lazy in the direct child position
of a `<Suspense>` or `<Activity>` component.