OrientInstanceReference: improved handling of unresolvable references

Change-Id: I87997318dda083aeed384464a64a161581a59ab6
Reviewed-on: http://gerrit.esdi-community.eu/3105
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>