Fix error messages when importing .keystone/config.js

[marekryb]

Fixes #9358

I brought back fs.stat that was removed in this commit.

Now error message is meaningful:

(base) exar@exar:~/os/keystone/examples/omit$ pnpm build

> @keystone-6/example-omit@ build /home/exar/os/keystone/examples/omit
> keystone build

🚨 importing built keystone config failed
Error: Cannot find module 'non-existing-module'
Require stack:
- /home/exar/os/keystone/examples/omit/.keystone/config.js
- /home/exar/os/keystone/packages/core/src/scripts/utils.ts
- /home/exar/os/keystone/packages/core/src/scripts/cli.ts

[codesandbox-ci]

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[dcousens]

Thanks @marekryb 💛