Let the generic error handler generate json again

app/templates/errors/error.html.j2

@@ -4,7 +4,7 @@
 
 {% block page_content %}
 <div class="container">
-  <h1 id="title">{{ error.name }}</h1>
+  <h1 id="title">{{ error.code }} {{ error.name }}</h1>
   <p>{{ error.description }}</p>
 </div>
 {% endblock page_content %}