Fix: Suspend while recovering from hydration error (#28800)

Fixes a bug that happens when an error occurs during hydration, React
switches to client rendering, and then the client render suspends. It
works correctly if there's a Suspense boundary on the stack, but not if
it happens in the shell of the app.

Prior to this fix, the app would crash with an "Unknown root exit
status" error.

I left a TODO comment for how we might refactor this code to be less
confusing in the future.