Support return error code

[lgarithm]

The operator() member functions are specified as two overloads, with and without an argument of type error_code&:

R f(A1 a1, A2 a2, ..., AN aN, error_code& ec);
R f(A1 a1, A2 a2, ..., AN aN);

The first one should not throw except, the second one should call the first by passing args through.