Overhaul the error reporting model

    This replaces TIPPLoadStatus with TIPPErrorHandler, which is
    provided to the factory before creating a TIPP.  The default
    handler (DefaultErrorHandler) throws exceptions on FATAL and ERROR
    errors, and ignores WARN.  A CollectingErrorHandler, which is
    analagous to the old TIPPLoadStatus, is also provided (and is
    used extensively in the unittests).