def configured_service!
      if s = configuration[:service] and services.keys.include?(s)
        return s
      else
        raise(
          Geocoder::ConfigurationError,
          "When using MaxMind you MUST specify a service name: " +
          "Geocoder.configure(:maxmind => {:service => ...}), " +
          "where '...' is one of: #{services.keys.inspect}"
        )
      end
    end
def configured_service!
      if s = configuration[:service] and services.keys.include?(s)
        return s
      else
        raise(
          Geocoder::ConfigurationError,
          "When using MaxMind you MUST specify a service name: " +
          "Geocoder.configure(:maxmind => {:service => ...}), " +
          "where '...' is one of: #{services.keys.inspect}"
        )
      end
    end
Return the name of the configured service, or raise an exception.