REF: use label/ as default for path to template (and check folder existence)
REF: use label/ as default for path to template (and check folder existence)

Former-commit-id: cfb30576226086d1eeb7da9a33280c2f747dc3b6
Former-commit-id: 0159830a03ee7551af8776514b717bda02a83352
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox