fix: add debug output to loader.js

Was almost impossible to debug where the loader errors were coming from when upgrading storybook with mdx2, until I monkey patched it with this change.

loader.js

@@ -23,6 +23,7 @@ const loader = async function (content) {
   try {
     result = await compile(content, options);
   } catch (err) {
+    console.error('Error loading:', this.resourcePath)
     return callback(err);
   }