Follow up: enhance UX when 404 cname is used at YOCDN

[Mai-Saad]

Currently

• When cname is 404, it isnot applied to FE and transient (24h) created => no notice displayed at admin
• While external requests blocked, user add valid cname, it won't be applied to FE (for 24h), if requests are back before 24h , cname will still not apply even if it was rewritten in the field and save settings => transient isnot deleted when cname settings changed

Expected

• Notice is displayed when cname isnot applied to FE (need copy)
• cname transient is deleted with save settings of cname