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/app.html

@@ -1,22 +1,9 @@
-{% extends "base.html" %}
+{% extends "unauth.html" %}
 {% from "library.j2" import avatar with context %}
 {% block head_lead %}
 <title>{% trans %}Snikket Web Portal{% endtrans %}</title>
 {% endblock %}
-{% block style %}
-<link rel="stylesheet" type="text/css" href="{{ url_for('static', filename='css/app.css') }}">
-{{ super() }}
-{% endblock %}
-{% block body %}
-<div id="topbar" class="{% block topbar_classes %}{% endblock %}">
-	<header><a href="{{ url_for('.index') }}"><span>{{ config["SNIKKET_DOMAIN"] }}</span></a></header>
-	{% block topbar_left %}{% endblock %}
-	<div class="filler"></div>
-	{% block topbar_right %}{% endblock %}
-	<nav class="usermenu">{{ user_info.display_name }}{% call avatar(user_info.address, user_info.avatar_hash ) %}{% endcall %}</nav>
-</div>
-<div id="mwrap"><main>{% block content %}{% endblock %}</main></div>
-<footer>
-	<ul><li>{% trans about_url=url_for('main.about') %}A <a href="{{ about_url }}">Snikket</a> server{% endtrans %}</li></ul>
-</footer>
-{% endblock %}
+{% block topbar_right %}
+{{- super() -}}
+<nav class="usermenu">{{ user_info.display_name }}{% call avatar(user_info.address, user_info.avatar_hash ) %}{% endcall %}</nav>
+{%- endblock %}