Keep the export running after seeing errors

At some point calling session.getSourceDocument(docRef) (and maybe others) can fail.
The point is that if when calling a method on the core it creates a runtime exception (like NPE), the container will kill the bean and we will end up with the error can not find statefull bean (without knowing the cause of the problem).

I think the problem is that in 5.1.6, getSourceDocument implementation does not catch all errors. This has been done in recent core version is order to avoid this problem.

• provide a patched version of DocumentManagerBean that is safe for getSourceDocument (and other methods)
• be sure the exporter code will just log errors when versions are missing but will continue