COObjectGraphContext (-objectReferenceWithUUID:): Throw a more descriptive exception when loading a graph with a broken reference (no functionality change)

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9981 7b3f36aa-e6db-49c1-8b4c-66eac1997e64