This fixes an edge case where an application can seemingly randomly be opted out of the automatic static optimization from having an `_error` file that isn't directly nested under the `pages` folder e.g. `pages/hello/_error.js`.
Since in serverless mode we use the `_app` export from the first serverless page, if this `_error.js` file is used to check for custom `getIniitalProps` in `_app` and the `_error.js` page had a custom `getInitialProps` itself it would cause the check to fail opting the application out of the static optimization.
This fixes it by having the check be explicit instead of relying on the bundle name and it also adds regression tests for this. It might be good to also add a warning when `_error` or `_app` are not directly nested under the pages folder in a follow up PR
