verify_token should return True or False, not raise HTTPError

[stefanoborini]

The signature of Hub.verify_token is a bit inconsistent. I suspect we use it for returning a reason for the failure, but it just looks weird, it might need some thought.

[kitchoi]

In jupyterhub, it is okay to raise HTTPError because their BaseHandler has overwritten the write_error and render an error page with the HTTPError messages. We could do the same with write_error.
As for successful request, it should return the json data instead.