[IMP] website_sale: show alert (error) message if VAT number, Email is invalid.

Give a proper error message, if users enter invalid VAT number as well as check email address is in valid format. 

<b>Task:</b> https://www.odoo.com/web#id=9970&view_type=form&model=project.task&menu_id=3942&action=327
<b>Pad:</b> https://pad.odoo.com/p/r.5xdH8OsRKzEJJzl9