Unlock place manager when async calls fail. 

Related group posting: https://groups.google.com/forum/#!topic/gwt-platform/3VDf-eHzBi0

There's also another place where 

    c.g.m.c.proxy.Proxy.getPresenter(NotifyingAsyncCallback)

is called: 

    c.g.m.c.proxy.RevealContentHandler.onRevealContent(RevealContentEvent)

Not sure whether there should be the same error handling.