“ The whole constitution of prize courts, taken in connection with the conclusive effect of their sentences . . . and the exercise of their functions, being admittedly a belligerent right, is an anomaly in jurisprudence.. . . Hence it is that while its authority is derived exclusively from one nation, and that a belligerent, it passes a conclusive sentence on the title to property belonging to another, and that a neutral state, whose only appeal in the last resort is through diplomatic inference to the justice of the sovereign of the captor.”