A wealthy New York organization that poured $15 million into last year’s unsuccessful ballot question to expand charter schools in Massachusetts was hit Monday with the largest fine in state campaign history after officials found the group was illegally hiding the identities of its donors. Families for Excellent Schools-Advocacy, a nonprofit that was the single largest funder behind Question 2 in Massachusetts, was slapped with a $426,466 fine, the largest in the 44-year history of the state Office of Campaign and Political Finance.