Comment 18 for bug 1611737

> This makes some upgrades technically not possible since there could not be packages in the ROS repositories available for the new distribution in an upgrade process. For example, there is no a single ROS Release that support 14.04 and 16.04. I can not see a safe/straightforward upgrade path.

Jose hit this on the head. This is explicitly not supported by ROS. The problem is, Ubuntu doesn't really have a way of knowing that. I think the best solution to this issue is a nice error message from the upgrader. We can detect that ROS is installed, mention that an upgrade isn't supported, and suggest removing the ROS distro before proceeding.

That said, I'm not familiar with the upgrader. Is there supported way we could hook such logic in?