Use correct heading class and better wording on error pages

app/templates/error/500.html

@@ -3,8 +3,10 @@
 {% block fullwidth_content %}
     <div class="grid-row">
       <div class="column-two-thirds">
-        <h1>500</h1>
-        <p>Sorry, something went wrong on our system.</p>
+        <h1 class="heading-xlarge">
+          Sorry, we're experiencing technical difficulties
+        </h1>
+        <p>Try again later.</p>
       </div>
     </div>
 {% endblock %}