code: public function errorOn($method, $error = 'A mock error', $args = false, $severity = E_USER_ERROR)
      {
          $this->dieOnNoMethod($method, 'error on');
          $this->actions->register($method, $args, new SimpleErrorThrower($error, $severity));
      }
docs: Sets up a trigger to throw an error upon the method call.
  
  @param string $method   Method name to throw on.
  @param object $error    Error message to trigger.
  @param array  $args     Optional argument list filter.
  If given then the exception
  will only be thrown if the
  method call matches the arguments.
  @param int    $severity The PHP severity level. Defaults to E_USER_ERROR.