If root FS is on volume not supported by GRUB2 - `update-grub` silently fails because `snapshots-btrfs` GRUB hook
silently fails at `root_fs=$(${grub_probe} --target="fs" /)` due to `set -e` at the beginning of `snapshots-btrfs`.
Failure of commands in test clause in Bash does not trigger set -v error and can be handled in script to
exit gracefully - implement this.
Fixes https://github.com/Antynea/grub-btrfs/issues/222
