Diagnose failures to load images in run sessions

I've seen a few cases now of users believing the only way to get images to display in CSIDE is via "Image Scenes".
This is not true: images work as they would in any CS game via the \*image command.

That said, CSIDE admittedly doesn't do anything to diagnose missing images... So it can be rather tricky to troubleshoot incorrect paths. We should fix that, maybe by logging issues to the project on any failure to load (404s) files with an image extension.

Additional context: https://forum.choiceofgames.com/t/header-image-not-showing-up/86180/7?u=cjw