SQL> create table t (id integer);

Table created.

SQL> create or replace trigger my_trig before insert on t
  2  begin
  3    raise_application_error(-20000,'Dont do that');
  4  end;
  5  /

Trigger created.

SQL> declare
  2    myexception exception;
  3    pragma exception_init(myexception, -20000);
  4  begin 
  5    insert into t values (0);
  6  exception
  7    when myexception then null;
  8  end;
  9  /

PL/SQL procedure successfully completed.

SQL> select * from t;

no rows selected

Insert Into t
Values (1);

CREATE OR REPLACE TRIGGER EMP_TRIG
  BEFORE INSERT OR UPDATE ON TSDETL 
    REFERENCING NEW AS NEW OLD AS OLD FOR EACH ROW 
      WHEN ((NEW.empno > 99))
DECLARE
  incorrect EXCEPTION;
  aa boolean;
BEGIN
  If :NEW.empno > 99 Then
     :NEW.sal := :NEW.sal + (:NEW.sal*5)/100;
  Else
     raise incorrect;
  End If;
Exception
  When incorrect Then
       raise_application_error(-20101,'Prevent it...');
END;