Bug 2047: Automatically retry when S3 transfer fails with 503 code (Slow down or Service unavailable)

https://winscp.net/tracker/2047

Source commit: c1b0ad797325e9c203340b7c2f72fc67c27ffe23

source/core/S3FileSystem.cpp

@@ -494,6 +494,7 @@ void TS3FileSystem::CheckLibS3Error(const TLibS3CallbackData & Data, bool FatalO
         break;
 
       case S3StatusConnectionFailed:
+      case S3StatusErrorSlowDown:
         FatalCandidate = true;
         break;
     }