Update URLs as per indications on cdash.
Fix an issue when GDCM is passed an invalid DICOM file

Thanks to Alexandre Fusellier <alexandre.fusellier@bioclinica.com> for report and test cases. All code paths were tested except in one case. Behavior now match CanRead contract.
DataElement::GetValueAsSQ throws exception instead of returning nullptr

See BUG: 331 for more detail.