Permits upgrade from an LTS system on an HWE stack to a broken system

See https://lists.ubuntu.com/archives/ubuntu-desktop/2013-November/thread.html for the discussion the led me to file this bug.

Steps to reproduce:

1. Install 12.04 with an HWE enablement stack, eg. with the 12.04.2 installer.
2. In update manager, choose to show upgrades to all releases, not just the LTS.
3. Upgrade to Quantal.

Expected results:

Since an upgrade from Precise with an HWE stack installed to Quantal is unsupported, the upgrade should either not be permitted from the GUI, or the user should be prompted to downgrade from the HWE first, or some other dire warning should be presented.

Actual results:

An upgrade is permitted. On reboot, the system has an HWE xorg from Precise that has not been upgraded, and the Quantal xorg release is not installed. On a system with an nvidia card, this completely broke the system and presented me with a black screen on reboot.

Critical Importance justification: Enabling an LTS to non-LTS release through the GUI is the standard recommended process to upgrade on a six-monthly cadence, and is valid on a non-HWE system. The HWE stack is default on the latest LTS point release, so users may not be aware of what the HWE stack is or if they are on one. So users can be led into a completely broken system just on the basis of timing of the latest installation image that gets chosen and timing of when to upgrade. I have heard general stories about systems being broken on upgrade, and as an Ubuntu developer was led into this myself. Given the "default" nature of this problem, I believe that it's reasonable to assume that this is an issue that has affected a large proportion of Ubuntu users.

Perhaps most users who could be affected by this have already been affected, in which case I'd understand a Won't Fix conclusion to this bug. The "default" window is still open though, as it's reasonable for a user to want to upgrade to 12.04 to 13.10 as 13.10 is still current. The pull to upgrade to a 6-monthly release grows as time goes on, since the benefits build up. As an example, this happened to me because I found that a 12.04 system didn't have certain packages I wanted, and so I considered it worth upgrading at that point.