ui: guest backup grid: monitor store errors

so that users see when something went wrong.

Don't mask the grid directly, but rather only its view so that the
title bar can be used for selecting another storage during load.

Reported in the community forum:
https://forum.proxmox.com/threads/get-backups-from-proxmox-randomly-fails.98383/

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

www/manager6/grid/BackupView.js

@@ -89,6 +89,8 @@ Ext.define('PVE.grid.BackupView', {
 		url: url,
 	    });
 
+	    Proxmox.Utils.monStoreErrors(me.view, me.store, true);
+
 	    reload();
 	};