Avoid warning: FacesMessage(s) have been enqueued, but may not have been displayed

[helkv]

Several imeji actions which show status messages (actions like: login/logout, saving/creating a collection, ...) cause a warning in the imeji log:

• "FacesMessage(s) have been enqueued, but may not have been displayed"
  ("FacesMessage(s) wurde(n) in die Warteschlange gestellt, aber möglicherweise nicht angezeigt.")

The warning is not triggered by all actions which show status messages (actions like upload items do not trigger the warning) and the warning is not logged on the server logs (it is only shown on local logs).

Enhancement: The warning should not occur, if no messages get lost.