Adds 404 and 500 pages

they only show up when debug is off though, thankfully
Fixes #112

fedireads/templates/notfound.html

@@ -0,0 +1,9 @@
+{% extends 'layout.html' %}
+{% block content %}
+
+<div class="content-container">
+    <h2>Not Found</h2>
+    <p>The page your requested doesn't seem to exist!</p>
+</div>
+
+{% endblock %}