@Override
    public Result search(Query query, Execution execution) {
        ErrorMessage error = validate(query);
        if (! isStreaming && error != null)
            return new Result(query, error);
        return execution.search(query);
    }
Check sorting specification makes sense to the search cluster before
passing it on to the backend.

@author Steinar Knutsen
container-search/src/main/java/com/yahoo/prelude/searcher/ValidateSortingSearcher.java