Comment 7 for bug 1307646

This looks like a duplicate of bug #1241395, but the proposed fix may not be appropriate for backporting to havana since heat havana didn't have the required clean up of formatting exception details in a json response.

https://review.openstack.org/#/c/87138/

It might be worth backporting this change to havana horizon, using a recent python-heatclient, and see if that improves the situation.