Adding addiontal logging in EtchPlainMailboxmanager

Fixing a TODO by adding logging in case the session object of the
mailbox manager is NULL.

Change-Id: I9e0374bf9a61bc3b3cd2ded5e7b5cc652514062c

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1578892 13f79535-47bb-0310-9956-ffa450edef68