Flush warnings from this method only to ensure stack level is right.  Also fix the message that I broke earlier by changing the implementation.

refs #7653


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/deprecate-jelly-instance-support-7653@43243 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb