You can write a custom actor which validate your submission then send cancel signal and message. Please see the doc here: https://world.episerver.com/documentation/developer-guides/forms/implementing-a-customized-actor/
Note: It is available from Forms 4.16
Thanks for this input! I will give it a try to see if it fits within our solution.
We're currently building custom elements in EPI Forms. Part of that is that we also want to call custom webservices with custom logic. Because we use multi step forms and only want to submit the data at the end of the process to the webservices, we can't use the FormsSubmitting event (that is triggered each step transition and only contains data of that specific step). Therefore we're hooking into the FormsSubmissionFinalized event which is triggered when the submit button is pressed. However the FormsSubmissionFinalized event doesn't have a cancellation handling mechanism like the FormsSubmitting event has. Is there a way to handle errors in that specific event, such that we can send feedback to the UI in case something is going wrong in the webservice calls?