[not ready] stats warnings

fix https://github.com/mapbox/mapbox-studio/issues/1016

Todo:
- Code/style cleanup
- Consistify UI with print warnings
@yhahn I thought it'd be nice to hook into existing UI components for warning reporting, so I am usng `errorHandler` here - Not sure this is a safe way to go technically. From UX point of view, this makes warnings pretty prominent, not sure yet if that's a good thing or a bad thing. Alternative would be to add a new component inside the `#zoomedto` div.