Handler which is called in case an async call fails. This can happen when a presenter behind a split point should be revealed and there's a network failure. In this case it is essential to unlock the placemanager.
Before navigation the placemanager fires a 'lock' event (see {@com.gwtplatform.mvp.client.proxy.LockInteractionEvent}) which causes the {@com.gwtplatform.mvp.client.RootPresenter} to insert an invisible 'glass' container in front ofall other elements (to prevent user events during navigation). When the place was revealed a matching 'unlock' event is fired. In case of an unsuccessful async call this 'unlock' event will not be fired and the 'glass' will not be removed. This gives the impression that the UI has crashed.
So this handler displays an error message, unlocks the placemanager and reveals the last successful place again.
@author Harald Pehl
@date 03/20/2013