"Not all updates can be installed" dialog launches distribution upgrade, not partial upgrade of packages

[Kieran O'Neill]

1) Ubuntu 12.04.2 LTS (with some PPAs); this behaviour has been present since at least Lucid.

2) update-manager 1:0.156.14.11

3) When an error occurs with one or more PPA packages, update-manager produces the "Not all updates can be installed" dialog, giving the options "Partial upgrade" and "close". If "partial upgrade" is clicked, update-manager commences a distribution upgrade to the next (non-LTS) distro version.

4) Since /etc/update-manager/release-upgrades contains the Prompt=lts setting, I would presume that update-manager should never try to upgrade unless a new LTS is available. I also would not expect PPA errors to cause update-manager to initiate a distribution upgrade at all. What I would expect update-manager to do would be to update those packages without error (which is what happens after you click "Close" on the dialog, and then click "Install Updates".