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

https://winscp.net/tracker/2047
(cherry picked from commit 00f7c85f2fdc4aec95e506416b0331437b54c99c)

Source commit: 17656959b50d686f01cfe431387c5d2366b8854c

source/core/S3FileSystem.cpp

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