[keyserver] Add descriptive error if assets.json can't be found

Summary: Noticed we had a descriptive error if the Webpack build couldn't be found, but not for assets.json.

Test Plan: Tested when assets.json was missing to make sure the new error was printed

Reviewers: rohan, atul

Reviewed By: rohan

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5351