Added templates for http exceptions.

dav_events/templates/500.html

@@ -0,0 +1,6 @@
+{% extends "error_base.html" %}
+
+{% block error-code %}500{% endblock %}
+{% block error-title %}Internal Server Error{% endblock %}
+{% block error-heading %}Serverfehler{% endblock %}
+{% block error-body %}<p>Irgendwas ist jetzt kaputt.</p>{% endblock %}