Improving management of snapshots and of the content of Lastest_Snapshot_URL and Lastest_Snapshot_Path commands

[ripleyXLR8]

Commands Lastest_Snapshot_URL and Lastest_Snapshot_Path contain URL and path to the lastest snapshot. But if the snapshot has been deleted, then the picture cannot be displayed in the widget and an error 404 occurs.

1. We could implement a way to manage commands value during the Ajax call to remove the snapshot.

2. We could check if the file exist before displaying it in the widget and therefore removing the 404 error.