Added templates for http exceptions.

dav_events/templates/400.html

@@ -0,0 +1,6 @@
+{% extends "error_base.html" %}
+
+{% block error-code %}400{% endblock %}
+{% block error-title %}Bad Request{% endblock %}
+{% block error-heading %}Fehlerhafte Anfrage{% endblock %}
+{% block error-body %}<p>Irgendwas ist hier falsch gelaufen.</p>{% endblock %}