As usual, Wikipedia comes through. The game ends when the snitch is caught. Whoever has the most points wins. Thus, it is possible for a team to have scored more than 150 points more than the other team via goals and for the losing team to get the snitch and score 150 points and still lose. Of course, it seems like it would be stupid for them to have gotten the snitch at that point, since that means that they cut out time they could have used to catch up first.