REF: added verification of file when getting image dimensions in sct_utils
REF: added verification of file when getting image dimensions in sct_utils

Former-commit-id: 8827398fe826790a0b3c99467f9fbc2b4d8f8aca
Former-commit-id: f5edbd6fa38f015e7a3df23d20b2989a2b98f58d
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox