Add basic error pages

app/templates/error/404.html

@@ -0,0 +1,10 @@
+{% extends "admin_template.html" %}
+{% block page_title %}Page Not Found{% endblock %}
+{% block fullwidth_content %}
+    <div class="grid-row">
+      <div class="column-two-thirds">
+        <h1>404</h1>
+        <p>Sorry, that page doesn't exist.</p>
+      </div>
+    </div>
+{% endblock %}