I created a function to calculate total amount (shipping + subtototal) for customer If customer isn't cerated I want to see in the output error

DBMS_OUTPUT.PUT_LINE('IDCUSTOMER: ' || customer_id || 'This ID does not exist!');

But when I run code, I don't see in the output error. What I do wrong?

CREATE OR REPLACE FUNCTION calculate_sub_withoout_ship
                          (customer_id bs_basket.idcustomer%type)
  RETURN NUMBER IS
  sub_calc NUMBER := 0;

BEGIN
  SELECT SUM(subtotal + tax)
    INTO sub_calc
    FROM bs_basket
   WHERE idcustomer = customer_id;
  RETURN sub_calc;

EXCEPTION
  WHEN NO_DATA_FOUND THEN
    DBMS_OUTPUT.PUT_LINE('IDCUSTOMER: ' || customer_id ||'This ID does not exist!');
END;
/

SELECT calculate_sub_withoout_ship(1000) FROM dual;

I get NULL and I don't see error.