Fixes ModuleRegistry raising MissingPackageVersion
Fixes ModuleRegistry raising MissingPackageVersion

If no version exists, get_descriptor_by_name() is supposed to raise
MissingPackage, apparently.

celiafish/VisTrails,hjanime/VisTrails,celiafish/VisTrails,hjanime/VisTrails,celiafish/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,minesense/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,minesense/VisTrails,Nikea/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,minesense/VisTrails,hjanime/VisTrails,Nikea/VisTrails