The ZFS driver should raise proper errors when the ZFS utility is
missing or when there's no zfs partition active on the system. Raising the
proper errors make possible to silently ignore the ZFS storage
driver when no default storage driver is specified.
Previous to this commit it was no longer possible to start the
docker daemon in that way:
docker -d --storage-opt dm.loopdatasize=2GB
The above command resulted in an exit error because the ZFS driver
tried to use the storage options.
Signed-off-by: Flavio Castelli <fcastelli@suse.com>