[keyserver] Print error when failing to load Webpack-generated assets

Summary: I was investigating an issue with my local dev environment recently and would have found these logs useful. In the past I've added these same logs when investigating other issues, so I think it would be helpful to just leave them in going forward. It might also be helpful for investigating issues in prod in the future.

Test Plan: Confirm the logs were printed

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12230