Expose loopback request test failure when "Failed to fetch URL to validate"

[westonruter]

Feature description

As seen in this support topic, when a site is unable to perform loopback requests, this is not made immediately clear to the user. The error message does tell the user to verify the site can perform loopback requests:

But we could be something more proactive by actually invoking \WP_Site_Health::can_perform_loopback() (e.g. via Ajax) and tell them right there it is failing.

If the issue is indeed with loopback requests, then we should provide some more guidance about how to resolve the issue, not only for the AMP plugin to work properly but also for this critical health of a site.

This is related to #3789, which is to proactively warn when loopback requests are failing.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This will not be relevant if we move away from using loopback requests for user-initiated validation. See #4979.

Also, see #5099 for validation request checking to Site Health.

Related: #3789.

[westonruter]

Closing this because we've not had many reports of this and we intend to move away from loopback requests for most validation checks. See #4979.