Print descriptive error message when old import format used.

      # Reads in data from all .json files in directory

      def read(ver)

        ensure_correct_format(ver[:path])

        # Map list of files into pairs of (classname, members-hash)

        pairs = Util::Parallel.map(Dir[ver[:path] + "/*.json"]) do |filename|

          JsDuck::Logger.log("Importing #{ver[:version]}", filename)

          Logger.log("Importing #{ver[:version]}", filename)

          json = Util::Json.read(filename)

          [json["name"],  members_id_index(json)]

        end

        return Hash[ pairs ]

      end

      def ensure_correct_format(path)

        # Read first JSON file in import dir

        json = Util::Json.read(Dir[path + "/*.json"].first)

        unless correct_format?(json)

          Logger.fatal("Bad format for importing: #{path}")

          Logger.fatal("Export format changed in 5.0.0 beta 2.")

          Logger.fatal("Maybe you forgot to re-generate the exports with new JSDuck.")

          exit(1)

        end

      end

      def correct_format?(json)

        json["members"].is_a?(Array)

      end

      # creates index of all class members

      def members_id_index(json)

        index = {}