[448949] Have a specific Exception similar to
org.eclipse.emf.ecore.xmi.PackageNotFoundException

Add org.eclipse.emf.cdo.util.PackageNotFoundException to be thrown by
CDOSessionImpl.loadPackages().

Change-Id: Iea22434675e8021a1f6ecf3947bb8521085ab9fc
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=448949
Signed-off-by: Esteban Dugueperoux <esteban.dugueperoux@obeo.fr>