So you would be able to create a more specific error message to inform users about the recursive flag. The error would have to be changed to something like: 

new_from_details("AssociationNotEmpty", "user", "spaces in the organization. You may pass the recursive flag when removing the relation to handle this for you.")

However, this seems like a fairly lengthy explanation, but it could work. Or I could make a more specific error for this relationship and have a cleaner format. I can implement it either way you would like.