Customizer: Error message instead of blank screen

[alexander.rohmann]

When Customizer can't load due to an error, there's no indication as to what's going on. It would be nice if some information was presented instead of seeing a blank screen on the right side.

[celloexpressions]

We should definitely show an error if the preview fails to load (or reload after attempting to navigate to other pages in it), probably in addition to adding something like a loading indicator. @westonruter's current work may be able to address this point.

The other possibility here is a JS error that prevents the Customizer controls from loading at all now. We'd have to have the error message loaded from PHP and shown if the Customizer controls JS stops executing due to an error, but I'm not sure that that would be as feasible.

[westonruter]

Depends on a notification area: #35210

[chriscct7]

Dependency ticket (#35210) punted to Future release. Punting this

[westonruter]

This is largely implemented now in #30937. Now when there is an error when loading the preview you'll see the error message that the server outputs (assuming you have WP_DEBUG enabled). One exception is when doing a refresh, any such error will not be shown since the refreshed iframe won't be displayed. If there is going to be a notification of an actual error even without WP_DEBUG then this will depend in part on having a notification area (#35210). Technically how the customizer will know there is an error when loading the page is that it will never receive a ready message, and thus it will know that there was a problem loading the window. This could be due to a server PHP error, or a network error, or a JS error.

[westonruter]

Duplicate of #22037.