Creating Custom user prompt / error message from a FormsEventHandler Business Rule

Is it possible to create a custom user prompt / error message to pop up when a user tries to complete a Form and triggers a FormsEventHandler?

I have tried throwing an Exception as below but the problem is it includes additional information about the business rule error that is irrelevant to the end user and could cause confusion. Is it possible to create a cleaner message that just contains the message itself.