private ErrorMessage missingSearchChainsErrorMessage(List<String> errors) {
        String message = String.join(" ", new TreeSet<>(errors)) +
                                     " Valid source refs are " + String.join(", ", allSourceRefDescriptions()) +'.';
        return ErrorMessage.createInvalidQueryParameter(message);
    }
The name of the query property containing the source name added to the query to each source by this
container-search/src/main/java/com/yahoo/search/federation/FederationSearcher.java