Bug#685625: kfreebsd-image-9.0-2-amd64: segfaults when upgrading
Oh, and you would probably get more useful info by running:
sh -x $(which grub-mkconfig)
That shouldn't change your GRUB config either, as it will just print it
to standard output if you don't use the -o flag as update-grub2 does.
I would guess that when this failure happened, it wouldn't have replaced
your GRUB config yet, so I would *hope* your system is still bootable
(assuming the old kernel image is still there).