Unhandled errors are duplicated with React 16

Created by: gaearon

Now I see two separate errors for every exception in React 16. Probably because we throw it in a different context and then rethrow.

We should probably come up with some way to deduplicate them. In generally might be a good idea to always deduplicate identical errors since it's not helpful to switch between them anyway.

As we fix this we'll need to also test the overlay behavior for errors caught by boundaries. I'm not sure what the ideal behavior would be. We should probably always show our overlay I think.