…Of course, the error doesn’t come until after a few hours of downloading all of the new packages.
Yet another Ubuntu upgrade bug not solved in time for release. The details are in the bug report here. I don’t understand how this bug has been known since February and not fixed in time for release.
The workaround: Download and install the Ubuntu 12.04 versions of libtinfo5, libncurses5, and libncursesw5 (in that order). You will have unresolved dependencies until you install all three, but you should be able to do them one by one with dpkg -i. (Do not just double-click and use the graphical Ubuntu software manager; use dpkg -i from the terminal.) After they are installed, run the distribution upgrade like normal.