BUG: when user does not have vertebral_levels.nii.gz, script crashes. Now coming back to previous commit. Needs to be fixed in the future.
BUG: when user does not have vertebral_levels.nii.gz, script crashes. Now coming back to previous commit. Needs to be fixed in the future.

Former-commit-id: 304c4d99853ea42507b7930451e1658dff7f69ae
Former-commit-id: 66f1a462746faf1e2d7316164bce63c4fdc2f6ff
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox