php – Magento throws a new Mage_Payment_Exception 503 error

I'm working on a custom payment extension. When you save the payment method, some of the entries from the payment method selection step are checked.

if they are not valid, they will return

Throw new Mage_Payment_Exception ($ errorMessage, $ errorMessageField);

But when I save ajax request the payment method step, the answer says 503 error

This is the code for error handling

private function throwPaymentException ($ errorMessage)
{
// Since OSC does not display the errorMessage from the exception, we also save the error in the session. These
// So we can pick it up from the session and show it in the form.
$ _SESSION[self::VALIDATION_MESSAGE] = $ errorMessage;

// raise the exception
$ errorMessageField = & # 39; p_method_ & # 39 ;. self :: PAYMENT_METHOD_BILLINK_CODE;

Throw new Mage_Payment_Exception ($ errorMessage, $ errorMessageField);


}

Everyone knows why Site 503 displays errors after triggering the Mage_Payment_Exception, and displays the errors at the end of the font.

Thank you