def find_destination(destination_name)
      destination = @destinations[destination_name]
      if destination.nil?
        raise MessageDriver::NoSuchDestinationError, "no destination #{destination_name} has been configured"
      end
      destination
    end
def find_destination(destination_name)
      destination = @destinations[destination_name]
      if destination.nil?
        raise MessageDriver::NoSuchDestinationError, "no destination #{destination_name} has been configured"
      end
      destination
    end
Find a previously declared Destination
 @param destination_name [Symbol] the name of the destination
 @return [Destination::Base] the requested destination
 @raise [MessageDriver::NoSuchDestinationError] if there is no destination with that name