You can achive in PL/SQL by defining your own excpetions as
Code:
declare
invalid_num exception;
pragma exception_init(invalid_num, -01722);
......
begin
  numvar := 'MyNameIsBillyBob';
exception
when invalid_num then
       dbms_output.put_line('ORA-01722: PL/SQL: invalid number: variable numvar');
end;