Fix: Incorrect type of thenable passed to DevTools

Selective hydration is implemented by suspending the current render
using a special internal opaque object. This is conceptually similar to
suspending with a thenable in userspace, but the opaque object should
not leak outside of the reconciler.

We were accidentally passing this object to DevTool's
markComponentSuspended function, which expects an actual thenable. This
happens in the error handling path (handleThrow).

The fix is to check for the exception reason before calling
markComponentSuspended. There was already a naive check in place, but
it didn't account for all possible enum values of the exception reason.