When an image fails to load on the cover art tab, typically because it hasn't been processed by the IA yet, the broken image is supposed to be replaced by an error message saying "Image not available yet, please try again in a few minutes.".
In the edit history, the message is normally "We are unable to display history for this cover art. For a current listing of cover art, please see the release's cover art page."

In Opera and Firefox, I get the message. In Chrome, there's just a broken image.

I'm not linking to an example for the cover art tab, because those come and go. An example of an edit is https://beta.musicbrainz.org/edit/24822892