BUG: Rebuild subject hierarchy if resolve step fails

If the saved scene is invalid for some reason (in the scene attached to the error report there were empty unresolved items in the unresolved items list, and as they did not have valid parents, resolving them failed), then the subject hierarchy node is removed, which triggers re-building the hierarchy from scratch. The resulting hierarchy will be flat (except for the model and annotation hierarchies that can be rebuilt in SH), but at least there will be a valid subject hierarchy

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26032 3bd1e089-480b-0410-8dfb-8563597acbee