I have added a method called ActiveRecordHelper#error_messages_on. This method is meant to provide the same functionality and output as ActiveRecordHelper#error_messages_for, which ActiveRecordHelper#error_message_on does not do. error_message_on provides nice functionality, but it's output is quite different from that of error_messages_for. This new method is intended to bridge that gap.
Please note that the unit test code was mercilessly stolen from http://dev.rubyonrails.org/ticket/7212 which was submitted by Rich Collins. He deserves full credit for that excellent patch.