error_mapper: Don't redirect to ErrorController for pagecached errors.

This prevents a situation where we get a cached error (e.g. a 404) back
from the pagecache and then the error_mapper (called by the
ErrorDocuments middleware) sees that status code and redirects the app
to the ErrorController which then re-renders the error and inserts it
into the pagecache at the end of the request.

r2/r2/config/middleware.py

@@ -57,12 +57,15 @@ webob.util.status_reasons[429] = HTTPTooManyRequests.title
 
 #from pylons.middleware import error_mapper
 def error_mapper(code, message, environ, global_conf=None, **kw):
-    from pylons import c
     if environ.get('pylons.error_call'):
         return None
     else:
         environ['pylons.error_call'] = True
 
+    from pylons import c
+    if c.used_cache:
+        return
+
     if global_conf is None:
         global_conf = {}
     codes = [304, 400, 401, 403, 404, 409, 415, 429, 503]