NEW YORK (Reuters) - A comic gag turned into a gaffe for Democratic presidential hopeful Hillary Clinton and New York Mayor Bill de Blasio, triggering a social media storm that raged through Tuesday over what some said was their insensitivity to African-Americans. New York voters head to the polls next week for the primary election on April 19. While Clinton has strong support from black voters, some supporters worry that Democratic challenger Bernie Sanders has begun making inroads with them. Clinton and de Blasio took the stage on Saturday at the Inner Circle, an annual media roast of politicians that...