BibCatalog: revert to old BibCatalogSystem API

• Revert to old API for BibCatalogSystem class, i.e. return errors as strings and pass for noops, instead of raising exception that was introduced in 3267d797c389cd5e33d07b83376291969427cfc0, because not all clients of BibCatalog were updated to the new style yet.

Co-authored-by: Jan Aage Lavik <jan.age.lavik@cern.ch>