Improve logging message when closing a session of embedded repo.

Under an embedded repository environment as a web app, the following log messages are observed when the servlet container is stopped.

23.04.2010 09:18:25 WARN
[org.hippoecm.hst.core.jcr.pool.BasicPoolingRepository$SessionFactory.destroyObject():919]
Failed to log out session.
java.lang.reflect.UndeclaredThrowableException

Because the webapp embedding the repository could be stopped earlier than other webapps, it makes sense.
However, the error log message could be improved for better understanding.