I don't think it's an error at all unless the runner attempting to steal second then advances on to third as a result of the throw. So, if the CF or 2B/SS (whoever isn't covering second) is backing up the play and there is no further advance, then no error. In this case, it's simply a stolen base, just as if the throw had been perfect but late.
However, if the runner does advance, then I think the official scorer has a judgement call to make. If the throw should have been handled with "ordinary effort" then I think it's an error on the middle infielder. However, I would guess in most cases the throw is going to be really bad and the catcher will get the E.