If your sample contained a volatile impurity what value would be in error, the mass of the anhydrous salt or the mass of the water?

The percent would be higher than what it actually is. Since the impurity is in the anhydrous salt when initially weighing it, the initial weight will be higher than it should. Now when you heat the hydrate, you will not only lose water weight, but also the weight of the contaminate. This will make the calculation of the percentage of water greater than what it should be.