Enhancement: 502/Bad Gateway should be 410/Gone

This converts Deuce from the 502/Bad Gateway error over to the 410/Gone error

Previously 502/Bad Gateway was being returned under the argument that Deuce is a "gateway" to storage in the normal deployment use-case (e.g OpenStack Swift/Rackspace Cloud Files). However, it's not really a service error as the 5xx error codes are reserved for. After discussion the team agrees on 410/Gone instead; client are suppose to be able remedy this by re-uploading the block which will then restore everything dependent on the missing data.

I did not see anything in the API Tests to update.
falcon/exceptions.py should have this method, and be named like HTTP_410_Error. Then HTTPGone can inherit from falcon.HTTP_410_Error, and super() will be good.