Just to display part of the JSON in the error message, that code came from `current.py` in master.  Sorry it's kind of misleading since git has no way of tracking that I took those lines from that file.

See: https://github.com/ipython/ipython/blob/master/IPython/nbformat/current.py#L55

Just because it's in master doesn't mean it's the right behavior.  It seems okay with me, do you think I should change it so it doesn't show any of the JSON?