As the buses carrying European finance ministers left for a gala dinner in the Latvian capital on Friday night, one of the party hung back at the hotel and then wandered off alone into the dusk. Greece’s Yanis Varoufakis had other dinner plans, he said, after a bruising first day of meetings in Riga that underlined his isolation as he tries to avert national bankruptcy. While other ministers were feted by their entourages with food and warm clothing during the meeting in Riga, Varoufakis was seen alone at almost every turn, eschewing aides or any security detail.