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: 2fe17b55419474c69f612c8ebd236cbf0e72b9d4 [formerly 8827398fe826790a0b3c99467f9fbc2b4d8f8aca]
Former-commit-id: f5edbd6fa38f015e7a3df23d20b2989a2b98f58d
Former-commit-id: 838d2dd08b3d0d75577ae60b3ad0d15e8458b9ae
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox