Throw error if children entry can't be found, even if early access (#35452)

Co-authored-by: Peter Bengtsson <peterbe@github.com>

lib/create-tree.js

@@ -30,14 +30,22 @@ export default async function createTree(originalPath, rootPath, previousTree) {
     try {
       mtime = await getMtime(filepath)
     } catch (error) {
-      if (error.code === 'ENOENT' && filepath.split(path.sep).includes('early-access')) {
-        // Do not throw an error if Early Access is not available.
-        console.warn(
-          `${filepath} could not be turned into a Page, but is ignored because it's early-access`
-        )
+      if (error.code !== 'ENOENT') {
+        throw error
+      }
+      // Throw an error if we can't find a content file associated with the children: entry.
+      // But don't throw an error if the user is running the site locally and hasn't cloned the Early Access repo.
+      if (
+        process.env.NODE_ENV === 'development' &&
+        originalPath === path.join('content', 'early-access')
+      ) {
         return
       }
-      throw error
+      throw new Error(
+        `Cannot find a content file at ${originalPath}. Fix the children frontmatter entry "/${path.basename(
+          originalPath
+        )}" in ${path.dirname(originalPath)}/index.md.\n`
+      )
     }
   }