Make grub-install check for errors from efibootmgr

Make grub-install check for errors from efibootmgr

Code is currently ignoring errors from efibootmgr, giving users
clearly bogus output like:

        Setting up grub-efi-amd64 (2.02~beta3-4) ...
        Installing for x86_64-efi platform.
        Could not delete variable: No space left on device
        Could not prepare Boot variable: No space left on device
        Installation finished. No error reported.

and then potentially unbootable systems. If efibootmgr fails, grub-install
should know that and report it!

We've been using similar patch in Debian now for some time, with no ill effects.

Signed-off-by: Steve McIntyre <93sam@debian.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>