customize login error message
You can use function form_get_errors and function form_clear_error

function _custom_validation($form,$form_state){
  $errors = form_get_errors();
  if(!empty($errors)){
    $string_error = $errors['name'];
    if (strpos($string_error, 'unrecognized username or password') !== FALSE) {
      form_clear_error();
      form_set_error('name','The password entered is wrong. Please retry.');
    }
  }
}