Sometimes exceptions other than PaymentGatewayExceptions can occur in PaymentProcess->buildPaneForm(). Or even PHP errors. In that case, they are never catched and result in a blank page The website has encountered an error.
IMHO this results in a horrible user experience. As a client, I would feel very uncomfortable trying to buy anything else on a website that would handle errors that way.
For my client website, I have overridden PaymentProcess in order to catch any errors that can occur and try to display a relevant message to the customer. The key ideas being to not display a blank page as if the whole site had broken down ; but also to let the customer know that at this step his/her payment could potentially already have been successful. So we don't want him/her to try again before we can take the time to check what went wrong.
class MyModulePaymentProcessPane extends PaymentProcess {
public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
try {
return parent::buildPaneForm($pane_form, $form_state, $complete_form);
} catch (\Throwable $t) {
if ($t instanceof NeedsRedirectException) {
throw $t; // we don't want to block NeedsRedirectExceptions because they are responsible for redirecting the customer
}
Utils::logThrowableTracedMessage($t); // this is some custom function that logs the whole Stack Trace. Also the admin receives an email alert when an error is logged so we can work on it right away.
\Drupal::messenger()->addError("A message that lets the customer know that at this step his/her payment could potentially already have been successful. So we don't want him/her to try again before we can take the time to check what went wrong");
throw new NeedsRedirectException(Url::fromRoute('my_module.error_page')->toString());
}
}
}
I think this is particularly important at this step because the customer has entered his/her payment information and is waiting for confirmation. But more generally I think we should never display a blank page with The website encountered an error. So I would suggest we also make sure this never happens in any situation inside the whole Drupal / Commerce experience.