lib/fs: use WARN instead of ERROR log level for the message when NFS diretory removal temporarily fails

this is expected condition, so it is better to use WARN log level for it

lib/fs/dir_remover.go

@@ -82,7 +82,7 @@ func dirRemover() {
 		if sleepTime < maxSleepTime {
 			sleepTime *= 2
 		} else {
-			logger.Errorf("failed to remove directory %q due to NFS lock; retrying later", w.path)
+			logger.Warnf("failed to remove directory %q due to NFS lock; retrying later in %.3f seconds", w.path, sleepTime.Seconds())
 		}
 	}
 }