Diagram/Board.php - ignore mkdir errors

[CV-4685](https://chesscom.atlassian.net/browse/CV-4685)

After these changes, the errors reported in [CV-4685](https://chesscom.atlassian.net/browse/CV-4685) should be ignored. Not sure why these errors are happening

todo:
- [x] ignore mkdir errors
- [x] small docs fix