OPT: changed exception to error and declared default value for output
OPT: changed exception to error and declared default value for output

Former-commit-id: 6e0ee1b9277a6113bbf3006be83c4600cbf1193d [formerly 42c44cfe3aec054f5373b17d434ac6bfc99bfcf2]
Former-commit-id: 83888747982616c82a5fdf9c6bb3a69da7c7b868
Former-commit-id: c1b5fe2f4fa3c70a6744587566fa9930887a8b1a
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox