[KristjanESPERANTO]

When a stylesheet is missing, the browser currently reports a MIME-type error because it receives an HTML 404 page. That is misleading and makes the problem harder to understand.

This change returns a CSS-typed 404 for missing stylesheets and logs the stylesheet path in MagicMirror.

Before

Refused to apply style from 'http://localhost:8080/config/custom.css' because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Error on loading stylesheet: config/custom.css

After

Failed to load resource: the server responded with a status of 404 (Not Found)
Could not load stylesheet 'config/custom.css'.