This adds a proper error when we fail to load a user's `next.config.js` as the default node error can be confusing on its own and makes it seem like something failed internally in Next.js. We can expand on the included error doc added and potentially also add better syntax errors by parsing with acorn when we fail to load this file in the future.
<details>
<summary>screenshot</summary>
<img width="962" alt="Screen Shot 2021-08-13 at 21 46 09" src="https://user-images.githubusercontent.com/22380829/129432211-6d858062-eea0-4bb9-8725-5bde98bdc47b.png">
</details>
