the forward to error page will fail if some response content has already been flushed on the socket to the web client.
in other words you need to make your @PostConstruct fail before any content is flushed.
here are some tips I've gathered:
1) reference your FBs high in the JSP so @PostConstruct is invoked earlier - could be a simple <h:outputText value="" rendered="#{myFB.whatever}"/> or <s:subview id="myFB"/>
2) increase JSP response buffer size of your app server
3) add a ServletFilter around your URI which can catch the exception and do a redirect to error page
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