Can't include relative JS file from layout
It looks like this is all happening in Craft's template folder, where anything with an underscore—like your _layout directory and its contents—won't be accessible via URL.

You'll definitely want to adjust your project layout to keep Craft's templates together in /craft/templates, and any front-end resources (like your videojs.thumbnails.js, thumbnails.png, etc.) in the /public folder.

The Routing documentation fully explains what Craft does with any front-end URL it gets—it's a tad intense but should fully explain what happens with a request.