Check that the file exists

I have run into the following issue a few times now.
1) Create an SCSS file
2) Import it into a React component
3) Run jest.
4) Jest passes.
5) Delete the SCSS file, keep the import statement as-is in your React component
6) Run jest.
7) Jest still passes even though the file doesn't exist.
8) Build fails in CI/CD pipeline.

It would be nice if identity-obj-proxy could check if the file exists before proxying the object.

Reproduce the issue with an SCSS import in a React component, then run Jest after deleting the imported file. Inspect identity-obj-proxy's proxying behavior; done means Jest fails when the imported SCSS file does not exist, rather than allowing the CI/CD build to fail later.

Written by the indexing model from the issue text.