JS/Carousel - "Carousel is sliding"

Created by: nlemoine

Steps to reproduce

• Open: https://v4-alpha.getbootstrap.com/components/carousel/
• Open browser console
• Click quickly on "next" or "prev" arrow multiple times
• You will get some errors as shown above:

Issue source

I can't understand why you're throwing an error when next/prev methods are called which is likely to happen when navigating a carousel. As long as events don't get queued, I think that shouldn't happen. https://github.com/twbs/bootstrap/blob/v4-dev/js/src/carousel.js#L124