[Question] Handling API errors in finishers

[cweiske]

My custom finisher class sends data to an external API, which can fail. In that case I would like to notify the user about that and allow him to re-submit the form to try it again (like a validation error).

How can I do that with powermail 12.1.1?

Validation happens in FormController::checkCreateAction, which internally forwards to ::createAction when all data are valid.
Return values of finishers in createAction are not used.

Powermail's own SendParametersFinisher does not check the return value, so it can't be used as example.