OPT: address the case of problematic segmentations (e.g., non continuous segmentation can occur at the extremities) --> the program display a warning but doesn't crash (fix issue #694).
OPT: address the case of problematic segmentations (e.g., non continuous segmentation can occur at the extremities) --> the program display a warning but doesn't crash (fix issue #694).

Former-commit-id: eb8dd71dd27c1fc7aee0930e446c6f7fc4b01fa4
Former-commit-id: dd46cd7e59e20b4041b101729f82f630f09903af
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox