def verify_one_bookspec_exists!
      bookspec_files = Config.instance.find_all_bookspecs
      raise PlainInformative, "No `.bookspec' found in the project directory." if bookspec_files.empty?
      raise PlainInformative, "Multiple `.bookspec' found in current directory" if bookspec_files.count > 1
    end
def verify_one_bookspec_exists!
      bookspec_files = Config.instance.find_all_bookspecs
      raise PlainInformative, "No `.bookspec' found in the project directory." if bookspec_files.empty?
      raise PlainInformative, "Multiple `.bookspec' found in current directory" if bookspec_files.count > 1
    end
@return [void]

 @raise PlainInformative if no .bookspec file don't exists or there are too many