[catalog-server] Handle invalid and not found packages better.

[justinfagnani]

This handles a few cases better, and tests that the db state is what we expect:

1. Not found packages
2. Packages with no customElements field in package.json
3. Packages with a customElements field in package.json, but a missing file (something we see in the bootstrap packages)

[github-actions[bot]]

A live preview of this PR will be available at the URL below and will update on each commit. = The build takes ~5-10 minutes, and will 404 until finished.

https://pr1404---site-khswqo4xea-wl.a.run.app/

Warning Until our Cloud Run project is public, only authorized users can view the above URL. The easiest way to view it authenticated is to run the following proxy command and visit http://localhost:5453

(gcloud beta run services proxy --project=webcomponents-org-test --region=us-west2 --tag=pr1404 --port=5453 site \
& gcloud beta run services proxy --project=webcomponents-org-test --region=us-west2 --tag=pr1404 --port=6453 catalog)