On error React will detach whole app - need a better warning

Currently if there's an error, React will attempt to unmount the whole tree - which is correct in the normal React context (where it owns the complete DOM tree). Here, though, it means that app will be detached from the existing DOM, and it will not be so apparent to the user.

Need:
a) Better error warning about what has happened
b) Provide a hook to handle errors in a custom way