Comment 6 for bug 1784093

That's what I was saying in the last paragraph of comment 0 -- that we could make delete handle the case where only a build request exists and no instance mapping.

However, if that's the *only* thing we did to fix this bug, if build_request.create() succeeds and instance_mapping.create() fails, we get an instance that the user needs to delete (is it in ERROR state?). Whereas if we do the single transaction thing, we can avoid leaving an instance around if the scenario happens and avoid having to get users to delete instances in this case.