Boos rained down early and often Saturday night at Citi Field, where the Mets followed up their trade of ace Max Scherzer with a punchless performance against the last-place Nationals. Mets pitcher Carlos Carrasco surrendered eight runs — including four in the first inning — in his latest ugly outing, failing to get through three innings in New York’s 11-6 loss. A throwing error by Mets rookie third baseman Mark Vientos on the first play of the game set the tone, and a barrage of hits by the Nationals followed.