Fix error handling

Previously, some kinds of errors would throw nice and fun cascades
of exceptions.

We now have a nice, clean error page for 500 and 503 (backend
connectivity) errors which includes minimal debugging information
for productive setups and a traceback for development setups.

In any case, the full exception is logged to the log with an error
ID which is printed on the error page.

snikket_web/templates/internal_error.html

@@ -0,0 +1,6 @@
+{% extends "exception.html" %}
+{% block box -%}
+	<header>{% trans %}Internal error{% endtrans %}</header>
+	<p>{% trans %}The web portal encountered an internal error.{% endtrans %}</p>
+	<p>{% trans %}Please try again later and/or inform your Snikket instance admin.{% endtrans %}</p>
+{%- endblock %}