I don't think this is too much of a concern. Assuming the ad network doesn't go down very often, you could do something like "Unable to deliver content. This may be caused by an ad blocker". These kinds of "something is wrong either on your end or ours" messages are fairly common across the web and in this case doesn't leave the user feeling wrongly blamed in case of false positive (since the problem "may" be the ad-blocker).
