Fix repository rollback when issuing a unchecked exception at document creation

Steps to reproduce: deploy addon nuxeo-platform-error-web.
At root of workspaces, new actions are displayed that perform a document creation followed by errors.
The one issuing a ClientException after the creation does not rollback the transaction (as expected), but the one issuing a NullPointerException tries to do it and does not rollback correctly. It appears that transaction fails, but that document is created anyway (restarting the server, it is here).