update request.invoke_exception_view to set request.exception

[mmerickel]

Currently request.invoke_exception_view restores request.exception to whatever it was before when it completes. I think instead it should follow the behavior of the excview tween in #3029 and overwrite request.exception and request.exc_info if it manages to return a response. This will simplify the lives of most people who would want to use the method in trying to conform to this idea of having the request.exception match the response object it was used to create.