Defense let down a pair of Mets pitchers. Their own defense, that is. Costly errors by Tylor Megill and reliever José Castillo led to four unearned runs in the Mets’ 8-4 loss to the Tampa Bay Rays on Saturday night in Queens. Megill’s miscue was particularly back-breaking, as it prolonged a five-run fourth inning in which the Rays pounded the right-hander. The defeat clinched the Mets’ first …