Better 404 (and 403) pages

Fixes #1819

templates/404.html

@@ -1,13 +0,0 @@
-{% extends "error.html" %}
-
-{% block title %}
-  <title>Page Not Found · Quay</title>
-{% endblock %}
-
-{% block content %}
-  <h3>The page you're looking for doesn't exist!</h3>
-  <h4>
-    <p>This is somewhat embarrassing, isn’t it? It looks like there's nothing here.</p>
-    <p>You probably want to return to the <a href="/">main page</a>.</p>
-  </h4>
-{% endblock %}

templates/base.html

@@ -40,6 +40,14 @@
       window.__auth_scopes = {{ scope_set|safe }};
       window.__vuln_priority = {{ vuln_priority_set|safe }}
       window.__token = '{{ csrf_token() }}';
+
+      {% if error_code %}
+      window.__error_code = {{ error_code }};
+      {% endif %}
+
+      {% if error_info %}
+      window.__error_info = {{ error_info|safe }};
+      {% endif %}
     </script>
 
     {% for script_url in external_scripts %}