def missing_interpolation_argument_handler
      @@missing_interpolation_argument_handler ||= lambda do |missing_key, provided_hash, string|
        raise MissingInterpolationArgument.new(missing_key, provided_hash, string)
      end
    end
def missing_interpolation_argument_handler
      @@missing_interpolation_argument_handler ||= lambda do |missing_key, provided_hash, string|
        raise MissingInterpolationArgument.new(missing_key, provided_hash, string)
      end
    end
Returns the current handler for situations when interpolation argument
 is missing. MissingInterpolationArgument will be raised by default.