“It is no defence to a messenger at arms, that he has not injured or betrayed the interest of his employers, or to an agent bungling an act in which adverse parties are concerned as a loan, that he was employed by one of them as by the granter of the bond. He acts for both parties, and the person injured, or who is to receive the security, is he for whose behoof the law will interfere, and by whom the order is held to be given.”