Return an error when no entities found for listing

We have a few commands that list things e.g credentials, clusters, buckets etc. At the moment these just return an empty list when there are none of the relevant entities. Instead we should take the approach that the columnar clusters command takes and return an error not found, to make it clearer to the user what is happening.

Start by locating the commands that list credentials, clusters, buckets, and other entities, then compare their empty-result behavior with the columnar clusters command mentioned in the issue. Run each listing command with no matching entities and verify that it returns a not-found error rather than an empty list.

Written by the indexing model from the issue text.