Add custom error pages

app/templates/500.html

@@ -0,0 +1,8 @@
+{% extends "base.html" %}
+
+{% block app_content %}
+    <h1>An unexpected error has occurred</h1>
+    <p>The administrator has been notified!</p>
+    <p>If he gets too many notifications, we might replace him with an AI</p>
+    <p><a href="{{ url_for('index') }}">Back</a></p>
+{% endblock %}