title: Can Ruby on Rails's respond_to return a line when the format is not supported?
body: <p>A usual usage of <code>respond_to</code> is like</p>
  
  <pre><code>respond_to do |format|
    format.html
    format.xml { render :xml =&gt; @data }
  end
  </code></pre>
  
  <p>can it be made so that when the format is not supported (such as json or csv not being supported above), instead of returning nothing, return a text line saying "the format is not supported", or better yet, have it automatically report "only html and xml is supported"?  It can know only html and xml are supported by the existing <code>format.html</code> and <code>format.xml</code> lines there. (if possible)</p>