Dependency caching: small output improvements

[mbg]

This PR makes two small changes related to dependency caching to improve the Action's output during normal operation:

• We intercept ReserveCacheError errors when saving caches, since these are expected when a cache already exists for a given cache key. We log a message indicating this instead of propagating the error itself.
• We allow warnings from tryGetFolderBytes to be suppressed, which would otherwise result in check annotations for a workflow run. This occurs during normal operation, since we try to calculate the cache size for all folders that may need to be cached for a given language, whether they exist or not. If they do not exist, that's not a problem for us.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[aeisenberg]

Nice.