catch ExtremelyRare™ 502 response from CloudFront

happens occasionally when they can't reach the mojang upstream server

lib/networking.js

@@ -70,6 +70,7 @@ exp.get_from_options = function(rid, url, options, callback) {
         break;
       case 429: // this shouldn't usually happen, but occasionally does
       case 500:
+      case 502: // CloudFront can't reach mojang origin
       case 503:
       case 504:
         // we don't want to cache this