Increase timeout for validate URL requests and handle timeout errors

[westonruter]

Currently there is a 5 second timeout when doing a validate_url request:

https://github.com/Automattic/amp-wp/blob/a68c93fd62df93c6c79cf99d4917bb912e11b684/includes/utils/class-amp-validation-utils.php#L1426-L1432

Given the added overhead of locating sources and bypassing the post-processor cache (#959), it can happen that a request to validate a URL can timeout, especially when the stylesheets have not yet been processed and cached.

So not only do we need to increase this timeout, we also need to add error reporting for when validate_url returns with an error, including in bulk/inline re-check, during post save in classic editor, and doing a REST API POST/PUT. At the moment, such validation requests fail silently, meaning a user can be shown that there is no change to the improvement of validation errors.

[westonruter]

Also, if a validate AMP URL does a redirect, the post needs to be deleted in the same way as if the URL no longer has validation errors.