testing: sct_register_multimodal: shorter string, print failure craziness
testing: sct_register_multimodal: shorter string, print failure craziness

Former-commit-id: 398c9ac46d3368dd960b647f1fb38958e8684aa3
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox