Bugfix: show a red box if uploading a background image fails

When I deployed background images originally I made it so that when an upload fails you’ll see a big red box (instead of a grey box).

I broke this behavior accidentally a little while ago and didn’t notice: you would instead just continue seeing a grey box. But now it’s fixed! You’ll see a red box again.

Still not a particularly clear interface and I’ll make it clearer eventually, but at least it’s working as intended again.

Happy Shmepping ya’ll!