Replacement for deprecated TYPO3 ErrorpageMessage - error-handling

Question

After the upgrade of the extension tt_board to TYPO3 10 I stumble over deprecation #77164 - ErrorpageMessage and AbstractStandaloneMessage.
The official page only states that the new Fluid-based ErrorPageController class should be used instead.
However this controller class is just able to generate a HTML result string for me. But the second half of the job is missing. The HTML output must be shown somehow by TYPO3 in an error dialog raised by an exception.
Is there any simple solution for this?
Before:
use TYPO3\CMS\Core\Messaging\ErrorpageMessage;
...
$message = $languageObj->getLabel('error_no_permission');
$title = $languageObj->getLabel('error_access_denied');
$messagePage =
GeneralUtility::makeInstance(
ErrorpageMessage::class,
$message,
$title
);
$messagePage->output();
Now:
use TYPO3\CMS\Core\Controller\ErrorPageController;
...
$message = $languageObj->getLabel('error_no_permission');
$title = $languageObj->getLabel('error_access_denied');
$errorController =
GeneralUtility::makeInstance(
ErrorPageController::class
);
$content = GeneralUtility::makeInstance(ErrorPageController::class)->errorAction(
$title,
$message
);
Can I send this $content to a TYPO3 object which will do the error output for the extension?

Answer 1

In general you can show it an any possible way, but you can use flashMessages to show it in standardized kind.
Be aware that for the frontend you have to implement according CSS for the styling.
In fluid it's looking like this and is part of extensions created with extension_builder:
<f:flashMessages />
It might be possible that you have to implement it a bit different to loop about several messages. I never use it usually but I remember that implementation changed once and doubt that it's reflected by this kind of implementation.
Below you find the required details listed:
The implementation for flashMessages is described here:
https://docs.typo3.org/m/typo3/reference-coreapi/11.5/en-us/ApiOverview/FlashMessages/Index.html
The description of the flashMessage-ViewHelper you find here:
https://docs.typo3.org/other/typo3/view-helper-reference/11.5/en-us/typo3/fluid/latest/FlashMessages.html
The CSS you could probably take from bootstrap but reliable from the extension examples. In the CSS for the backend you could search it too.
Concerning your code:
I think you can leave away this, as it's not used:
$errorController =
GeneralUtility::makeInstance(
ErrorPageController::class
);
or you can use this code:
$errorController =
GeneralUtility::makeInstance(
ErrorPageController::class
);
$content = $errorController->errorAction(
$title,
$message
);