What functionality is missing? What is unsatisfying with the current situation?

Currently, whenever creating a custom statblock or article for coders, when testing to see if a page works, and it fails, all you get is a 404 error on the page. This means if you have a huge article or stat block, you have to comb through each line, editing, removing or adding until the 404 error dissapears. This results in a lot of traveling back and forth between multiple tabs in order to fix an error on a page.
Currently, there is the EUCLID editor for templates and pages that highlights the different BBC code options. Using this system as a base could help produce more accurate, or easier options for world anvil templates and blocks.  

How does this feature request address the current situation?

A debug menu that shows where an error is, or can occur, could help keep the time spent off of doing custom pages, and more time spent writing the world.  

What are other uses for this feature request?

Part of this feature is already in use with the EUCLID editor, but can also be expanded to work for other features potentially added in the future. One immediate example could be for maps, when someone inputs coordinates for a map marker that does not exist within the bounds of the image, and wouldn't be seen by anyone. The possibilities are endless.