It is a throwing error on the SS. OBR rule 10.12(d) says:
"(d) The official scorer shall not charge an error against: (1) the catcher when the catcher, after receiving the pitch, makes a wild throw attempting to prevent a stolen base, unless the wild throw permits the stealing runner to advance one or more extra bases or permits any other runner to advance one or more bases;
(2) any fielder who makes a wild throw if in the scorer’s judgment the runner would not have been put out with ordinary effort by a good throw, unless such wild throw permits any runner to advance beyond the base he would have reached had the throw not been wild;
(3) any fielder who makes a wild throw in attempting to complete a double play or triple play, unless such wild throw enables any runner to advance beyond the base such runner would have reached had the throw not been wild;
Rule 10.12(d) Comment: When a fielder muffs a thrown ball that, if held, would have completed a double play or triple play, the official scorer shall charge an error to the fielder who drops the ball and credit an assist to the fielder who made the throw."
The bolded section shows that it is an error, because the runner advanced. And the familiar notion that one "can't assume a double play" is shown to be not always true by the comment. In reality, "can't assume a double play" refers to the principle that getting the second out in a possible DP is difficult. So, for example if the throw is on time, but F3 is pulled off the bag, no error is charged, as long as the batter runner is held to 1st base. You can't assume the second out, unless the throw is accurate. But you can assume that the B/R shouldn't end up on second base.
