Custom validation field messages

Summary

Custom validation field messages

Content

Hi,

I've worked out how to do custom field validation, but instead of firing a message at the top of the page I wanted to show a validation message on the particular field I'm validating.

For example if you set a text field as required it shows the red validation message on that field. However I can't see a way to fire one of those messages on a field from custom validation.

For clarification, I'm validating a Buttonset Many and making sure the user is selecting at least one option. I've got valdiation working, but I can't see how to setup a per field validation message for it.