The Rails docs don't make it easy to find, but the answer is here: http://api.rubyonrails.org/classes/ActiveRecord/Errors.html
You can access any errors that come up through your model's object, and then in your RJS you format and return the messages. An example with a Post model might look something like this:
page.visual_effect :highlight, :message
unless @post.errors.empty?
@post.errors.each_error do |attr, err|
page.insert_html :bottom, :message, %(#{attr} #{err.message}<br>)
end
else
page.replace_html :message, "Huzzah! I posted!<br>"
end
where :message is the id of the DIV you're using to display the errors.