Check that the file exists

[mrbinky3000]

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.