Show SlowDown error message if backend is busy (#7521)

or if there are too many open file descriptors.

cmd/fs-v1-helpers.go

@@ -206,6 +206,9 @@ func osErrToFSFileErr(err error) error {
 	if isSysErrPathNotFound(err) {
 		return errFileNotFound
 	}
+	if isSysErrTooManyFiles(err) {
+		return errTooManyOpenFiles
+	}
 	return err
 }