lxd/db/storage/volume/snapshots: Comment and error improvements to CreateStorageVolumeSnapshot
lxd/db/storage/volume/snapshots: Comment and error improvements to CreateStorageVolumeSnapshot

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd