title: Raise 404 and continue the URL chain
body: <p>I've got a URLs pattern like this:</p>
  
  <pre><code>urlpatterns = (
      url(r'^$', list_titles, name='list'),
      url(r'^(?P&lt;tag&gt;[a-z\-0-9]+?)/$', list_titles, name='filtered-list'),
      url(r'^(?P&lt;title&gt;\S+?)/$', show_title, name='title'),
  )
  </code></pre>
  
  <p>The <code>filtered-list</code> and <code>title</code> match the same things.</p>
  
  <p>If there is an available list of things matching the <code>tag</code> in <code>filtered-list</code>, I want <code>list_titles</code> to fire off. But if there isn't a matching <code>tag</code>, I want to bubble that back to the URL processor so <code>show_title</code> fires off.</p>
  
  <p>If there's no matching title, I'll raise a proper 404 there.</p>
  
  <p>I know I can do this from inside the view...but it's a bit smelly having to hard-wire the process into the view. I'd like the URL order to decide what gets chosen first and what it hands off to.</p>