Improve handling of validation errors in Reader Mode

[amedina]

In Reader Mode validation errors are silently suppressed so users have no idea when something is being removed for being invalid.

Note: that validation error messages used to be present in Reader mode, but they were removed in #1132 because users were complaining about seeing undesired warnings (apparently ignorance is bliss).

The new Reader mode theme will need to trigger the standard theme hooks including wp_head and wp_footer so that scripts and stylesheets that a plugin may be trying to the page will be included and then reported as validation errors. This would have illuminated the reason for breakages such as ampproject/amphtml#26622.

Of course, there is still going to be the need to suppress validation errors from certain user roles or on certain site configurations (e.g. WordPress.com). This is needed for Standard/Transitional mode already.

It would also need to be available in Reader mode. See #2673.

[westonruter]

Closely related to #3821