don't allow upgrades to be left half finished

I stupidly set an upgrade running, ran "shutdown -h +30" and went out. Later, GNOME was exceptionally broken and I couldn't work out why (thinking it was a bad upgrade). Eventually when I ran apt-get it pointed out that I had an unfinished upgrade and I should run some dpkg command. Since lots of people are as dumb as me, I reckon this should be more prominent - before X starts the upgrade should be finished automatically, (with an option to skip if the user is doing something clever). Half-finished upgrades can clearly break gnome quite badly - none of the panel, metacity etc. would start so I basically had a black screen and a cursor there, which isn't very user friendly.