show additional info if btrfs subvolume deletion fails (issue #315)

show additional info if btrfs subvolume deletion fails (issue #315)

Unprivileged users require "-o user_subvol_rm_allowed" mount option for btrfs.
Make the INFO level message to ERROR to make it clear, which now says following;

[caglar@qop:~] lxc-destroy -n rubik
lxc_container: Is the rootfs mounted with -o user_subvol_rm_allowed?
lxc_container: Error destroying rootfs for rubik
Destroying rubik failed

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>