Absence of X-Cache-Channel will be enough for Varnish to skip caching

Do not override Cache-Control in this case, which means let the
clients or geographical proxies cache the response with usual TTL.

lib/cartodb/server_options.js

@@ -70,10 +70,6 @@ module.exports = function(){
               res.header('X-Cache-Channel', channel);
               cb(null, channel); 
             } else {
-              // avoid caching this result
-              // (temptative, what Varnish does is out of our control)
-              res.header('Cache-Control', 'no-cache,no-store,max-age=0,must-revalidate');
-              res.header('Pragma', 'no-cache');
               console.log('ERROR generating cache channel: ' + ( err.message ? err.message : err ));
               // TODO: evaluate if we should bubble up the error instead
               cb(null, 'ERROR');