Throw error if targets.json isn't found when adding a target.
Throw error if targets.json isn't found when adding a target.

advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr