Prevent row from being inserted with a trigger

I'm trying to prevent a row being inserted if a certain condition is met using a trigger. I'm using a user defined exception to do this, however the row is still being inserted, despite the exception being thrown. I'm not sure why this is happening, I though when an exception was raised the transaction was rolled back and nothing would be inserted. The code is below.

create or replace TRIGGER 
  CHECK_OPTION
BEFORE INSERT 
ON lot_options  
FOR EACH ROW

DECLARE
    v_allowed_stage NUMBER;
    v_actual_completion_date DATE;
    option_change_invalid EXCEPTION;
BEGIN
    DBMS_OUTPUT.PUT_LINE('Starting the begin');
    SELECT stage_stage INTO v_allowed_stage
    FROM "TEAM3"."Option" o
    WHERE o.option_number = :new.option_number;
    DBMS_OUTPUT.PUT_LINE(v_allowed_stage);

    DBMS_OUTPUT.PUT('Stage in a cell');

    SELECT actual_completion INTO v_actual_completion_date
    FROM progress
    WHERE stage_stage = v_allowed_stage
    AND Lot_lot_number = :new.lot_lot_number;
    DBMS_OUTPUT.PUT_LINE('completion date in storage');


    IF (v_actual_completion_date IS NOT NULL)
    --DBMS_OUTPUT.PUT('condition holds true')
    THEN RAISE option_change_invalid;
    END IF;

    EXCEPTION
    WHEN option_change_invalid THEN
    DBMS_OUTPUT.PUT_LINE('Invalid Option Change: Option change is not allowed in this stage of progress.');
END;