add in a 403 page

apps/amo/templates/amo/403.html

@@ -0,0 +1,18 @@
+{% extends "impala/base.html" %}
+
+{% block title %}{{ _('Oops') }}{% endblock %}
+
+{% block content %}
+  <section class="primary">
+    {% trans %}
+      <h1>Oops! Not allowed.</h1>
+      <p>You tried to do something that you weren't allowed to.</p>
+    {% endtrans %}
+    {% if csrf %}
+      {% trans %}
+      <p>Try going back to the previous page, refreshing
+      and then trying again.</p>
+      {% endtrans %}
+    {% endif %}
+  </section>
+{% endblock %}

apps/amo/views.py

@@ -190,14 +190,19 @@ def _paypal(request):
 
 
 def handler404(request):
-    return jingo.render(request, 'amo/404.lhtml', status=404)
+    return jingo.render(request, 'amo/404.html', status=404)
 
 
 def handler500(request):
     arecibo = getattr(settings, 'ARECIBO_SERVER_URL', '')
     if arecibo:
         post(request, 500)
-    return jingo.render(request, 'amo/500.lhtml', status=500)
+    return jingo.render(request, 'amo/500.html', status=500)
+
+
+def csrf_failure(request, reason=''):
+    return jingo.render(request, 'amo/403.html',
+                        {'csrf': 'CSRF' in reason}, status=403)
 
 
 def loaded(request):

settings.py

@@ -1160,3 +1160,5 @@ WATERMARK_REUSE_SECONDS = 1800
 # by a cron. Setting this far apart from the reuse flag so that we
 # shouldn't have an overlap.
 WATERMARK_CLEANUP_SECONDS = 3600
+
+CSRF_FAILURE_VIEW = 'amo.views.csrf_failure'