Rename frontend to legacy-frontend

legacy-frontend/pages/_error.js

@@ -0,0 +1,8 @@
+export default function Error(props) {
+  return (
+    <>
+      <h2>Oops, there is an error!</h2>
+      <pre>{JSON.stringify(props, null, 2)}</pre>
+    </>
+  )
+}