Show warning to users when they try to source the wrong files

Is your feature request related to a problem? Please describe.

When a user tries to source style.css (rather than styles.css) this fails silently. Similarly, sourcing anything other than script.js will result in a rather cryptic error: Uncaught SyntaxError: Unexpected token '<'.

This is needlessly frustrating.

Describe the solution you'd like

The console should show a helpful warning. For example

You have tried to source style.css, but that does not exist. The only files that can be sourced are styles.css and script.js.

Describe alternatives you've considered

Nothing comes to mind.

Additional context

No response