sct_label_utils: changed ERROR message for WARNING if label not found
sct_label_utils: changed ERROR message for WARNING if label not found


Former-commit-id: f75394b463cd605187f387113e577ffd1f25f94e
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox