hook_update_last_removed() not working properly

I tried to update a d6 database to d7 without having run system_update_6049(). update.php showed me an empty warning, and the updates for all other core modules - this allows you to try to update other modules (and get fatal errors, fun) - whereas you shouldn't be able to upgrade anything if system.module can't be upgraded itself.

We need to fix the empty drupal_set_message(), then also hide all updates for other modules from that page as well.