This a temporary workaround for addressing PLANET-6372

Because of the other bug, the Carousel Header gets incorrect data for the images. The source of the largest variant is the temporary image on the PHP server, while it should use the CDN URL.

This source is removed when we deploy, resulting in an empty image on big screens.

While this doesn't fully address the issue, it does prevent the worst effect (browser using 404 image).

We're also adding a blocking alert message in case the issue is detected, so that the editor in question knows it happens, and can contact the Planet 4 team with reproduction steps.

Relevant PR