Mary Catherine's Bakery   Although they're known for their extraordinary custom wedding cakes, Mary Catherine's, located in an odd, old building at the confluence of King and Waialae, also can do you a darn fine scone and coffee snack. They make a full range of baked goods and a rarity these days everything tastes as rich and delicate as it looks.  2820 S. King St. 946-4333. 9 a.m.-7 p.m. daily. MC, VS. Checks accepted. $.