Your application code running in the browser has to know how to support this. The browser doesn’t care about the shape of any JSON going or coming to/from an HTTP API.
Thanks, I don't know how I jumped to that conclusion re-reading now. Just wanted it to be the case I suppose - I think browser-integrated/'native' errors would be nice. I suppose a browser extension could do it, quite a permission to give though (I assume they can't request media type scoped permission to read responses).