Comment 2 for bug 1663172

One way I've been thinking about error vs. warning for u-i is how much work will be thrown away if we error out. If we've already called prepare-image to download the snaps, then erroring out throws all that work away (along with all the download latency). So post prepare-image, I tend to want to warn only. The bogus setting can be overridden by something that makes more sense. Before prepare-image then errors are more useful.

That said, if we can't override with something sensible, then it makes sense to error even after prepare-image.