For info: the motivation for this is to at least allow mrinfo to report the contents of a DICOM dataset even if the voxel data are stored in a compressed format that we can't currently handle (e.g. JPEG)

Actually, in retrospect, the documentation shows mrinfo failing due to unsupported transfer syntax, whereas with this change, mrinfo will succeed but others will fail. So that example should ideally be updated.

Found some unsupported data. Works as intended at my end, though the user feedback is perhaps sub-optimal:

• mrinfo issues the full warning, but then reports header information anyway, without any specific message about being able to read the header but not the intensity data;
• Any other command first issues the same warning, then reports as error "No suitable handler to access data". This is because the same Exception class is thrown regardless of whether a handler is incapable of accessing an image vs. there was some issue in the appropriate handler reading the image. Transitioning to STL standard exceptions (which we've discussed, but is not a C++17 thing, and doesn't yet have its own Issue) would potentially facilitate disambiguation here. Not a dealbreaker though.

This requires review by someone other than myself or @jdtournier as we can't approve our own changes to master.