doInstallStep: if there's not enough room, error, but packages are marked installed :(

when you click directly on setupFilesystems and you have scsi, you're fucked.
Even if you go to setupSCSI, you won't be able to have partitions again.

bug non reproductible:
- /etc/passwd is empty after `root passwd' stage :(
- reboot at the end of package installation with a lot of error messages from
gtk about the progressbar