add error handler pages for 400, 404, 500

incorporealcms/templates/400.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>BAD REQUEST</h1>
+    <p>You're doing something you're not supposed to. Stop it?</p>
+</div>
+{% endblock %}