Comment 4 for bug 1744325

I wouldn't say this is critical. It's a change, yes, but it's similar to what we always had as a bug when trying to rebuild a volume-backed instance with a different image up until Queens when we finally made that an error in the API because it's not supported in the compute service. I think we can just mark the instance ERROR in this case and let the user rebuild the instance with a valid image to get it out of ERROR state, but just leave the instance properties as the API set them without trying to get the conductor manager code to roll everything back.