Not long ago, we introduced custom models for webservices and for the the web endpoints. It is now possible to add validations to these custom models. Allowing you to build forms and flows that only save records when all fields have been filled in correctly.
Users will no longer be allowed to continue until they actually filled in a real email address or that terms-of-service checkbox has actually been checked by the user.
And what's more is that you can now show the user which fields didn't get past validations. Validations for custom models can be based on actual model properties, although manual validations were also made available.
Simply use the valid?() expression function in your condition events and proceed accordingly. You can then show the errors in your templates by using