Slicer SEG export is failing when reference volume is not DICOM

[fedorov]

At the very least, it should give an error message and not abort as it is right now.

Ideally, it probably would make sense to automatically export reference volume as DICOM, and then use for generating the SEG object. At this point, I don't know if that export to DICOM could be done completely automatically for the reference volume.