Removing a bundle from discovery might cause panic at eval-time

[johanfylling]

Given: a discovery bundle declares two bundles, a and b, where a depends on a function declared in b. If the discovery bundle is updated, and b is removed, a will fail to compile (as expected). This will however also have the following unexpected outcomes:

1. When evaluation is triggered via the /v1/data/ endpoint, a panic will be encountered in the topdown evaluator.
   The HTTP request is terminated with an empty response.
2. A request to /health?bundles will return a 200, and not the expected 500

This issue relates to #5917.