Ajax on failure message

How do i change the ajax failure message

  <% remote_form_for(:comment, :url => grocery_comments_path(params [:id]), :html => { :id => 'new_comment_form' }, :method => :post, :update => { :failure => "error" }) do |form| %>

the :update => { :failure => "error" }

gives me the usual error messages inside the <div id="error"> </div>

How do i change this error message to a string?