When upgrading fails (e.g. forced by root-partion of 80MB only), the system
remains in an unusable state:

- old rpm3 db was converted to rpm4, but no rpm4 available
- the stage2 image filled the root partition up to the last bit and so some
operations (mounting devices) got impossible

I think if upgrades fail some cleanup must be done. E.g. if something goes
wrong before installing the packages, the old rpm3 database must be
restored and temporary files be deleted.