Resolve "Error in 5xx pages"

Closes #494 (closed)

aleksis/core/templates/500.html

@@ -6,7 +6,7 @@
   <div class="container">
     <div class="card red">
       <div class="card-content white-text">
-        <div class="material-icons small left">error_outline</div>
+        <i class="material-icons small left">error_outline</i>
         <span class="card-title">{% trans "Error" %} (500): {% blocktrans %}An unexpected error has
           occured.{% endblocktrans %}</span>
         <p>