BUG: check that level file is an image before checking size and orientation. Fix #833
BUG: check that level file is an image before checking size and orientation. Fix #833

Former-commit-id: 849ba29b00dd98fde04ecdda4ff4bd70c556027c [formerly 9712db2325a70b4b6b3806ca48e79def423c0aab]
Former-commit-id: debe1f4b243c8888e3126a76ab1dbd725e70480b
Former-commit-id: 47d3ba0dc86a9140d10d77b97d74c866ec1b087d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox