Mark the session as failed. It will be terminated shortly afterwards and the setup wizard will end with the Installation Aborted page. No other error message will be given; it is up to your extension DLL to do so, if desired. Your extension DLL will receive a TIXMSG_ABORT_PROCESSING message, unless the session has already failed.
Note: If extension DLL errors are masked (as specified by the Error handling... option on the Installer options project page), then the failure request is ignored.