current exception state.  If
        C{_stuff} is an C{Exception} instance it will be wrapped in a
        L{Failure}.
    @type _stuff: L{None}, C{Exception}, or L{Failure}.

    @param _why: The source of this failure.  This will be logged along with
        C{_stuff} and should describe the context in which the failure
        occurred.
    @type _why: C{str}
    Né