A person is guilty of blackmail—
if, with a view to gain for himself or another or with intent to cause loss to another, he makes any unwarranted demand with menaces; and for this purpose, a demand with menaces is unwarranted unless the person making it does so in the belief—
(a) that he has reasonable grounds for making the demand; and
(b) that the use of the menaces is proper means of reinforcing the demand.