add error handler pages for 400, 404, 500

incorporealcms/templates/500.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+
+{% block header %}
+<div class="header">
+    <a href="/">{{ config.TITLE_SUFFIX }}</a>
+</div>
+{% endblock %}
+
+{% block body %}
+<div class="content">
+    <h1>INTERNAL SERVER ERROR</h1>
+    <p>Something bad happened! Please email me at bss @ &lt;this domain&gt; and tell me what happened.</p>
+</div>
+{% endblock %}