storage message "adding a field to a chidren" should be printed as a warn instead of an error

The following message logged by nuxeo core storage do not refer to an error.
It should be printed out as a warn instead of.

server.log.2009-12-07:2009-12-07 19:58:03,234 ERROR [org.nuxeo.ecm.core.storage.sql.Children] Adding already present id: 0a5a7297-8fbc-4b9f-a43d-4a11748625ae