Comment 0 for bug 1891680

Currently on upgrade if the debconf variable for the drive to install grub-pc to point to a non-existent drive, the grub package will nevertheless happily carry on and the postinst will exit 0 - as a result leaving the /boot/grub contents and the MBR in an inconsistent state, which due to recent ABI changes will leave the system unbootable on reboot.

Three changes required in order to make grub upgrades more resilient:

- exit non-zero from the postinst when the drive targets are invalid, so that we signal to the user that there is a problem BEFORE they reboot and give them the opportunity to deal with it. This is addressed by https://code.launchpad.net/~xnox/grub/+git/grub/+merge/388383
- include a check for target drive validity in the grub preinst, not just in the postinst, so that we avoid unpacking boot assets onto disk that might be incorrectly used by another package (despite grub-pc being in an unconfigured state) and still render the system unbootable; this will in general break release upgrades for affected users, but a failing postinst would do the same anyway, and failing early should leave the package manager in a more consistent state overall.
- modify grub-install so that it handles the flaky part of the install - updating the BIOS disks - FIRST, and aborts if this fails; instead of the current behavior, which is that /boot/grub is updated on disk first, then it attempts to install to the BIOS disk, and if this part fails, no rollback of the contents of /boot/grub is possible.