An executive from the largest Massachusetts health insurer has been chosen to oversee fixes to the state's failed health insurance website, the Patrick administration is expected to announce soon, in a move that one health policy specialist said signals a recognition that the pace of repairs is too slow. Sarah Iselin, who is chief strategy officer at Blue Cross Blue Shield of Massachusetts, will be charged with coordinating efforts among several state agencies and website developer CGI to get the site working again. Jean Yang, who will continue in her role as executive director of the Massachusetts Health Connector Authority, notified board members of the change this week.