Installation fails if alias domain no longer exists or pointed elsewhere

[timbutler]

By default all aliases for a domain are selected when generating the certificate. If one of these aliases has been pointed elsewhere or no longer exists, the certificate will fail to generate.

To compound the issue, the error displayed to the user in the Plesk admin is simply that the challenge failed. This doesn't identify the domain, making diagnosis of the issue very difficult.

Ideally, this extension should either indicate which domains the challenge is failing for and then indicate that the domain may not exist or may be pointed elsewhere. If the site is through a proxy or CDN (eg Cloudflare) this could produce a minor false positive, so it should only display if it fails to validate.