Hitting a non-existant mapping.json in a bower hosted repository returns a blank page. However, page debug shows "Failed to load resource: the server responded with a status of 404 (Not Found)" so the suspicion is that it's just not utilizing the 404 page we have in general place.
Example (with bower-hosted repository newly setup): http://localhost:8081/repository/bower-hosted/test2/mapping.json
I did not back check older NX3 versions. This bug does not affect NX2 as it this format is not implemented there.