function onServer( error, _server ) {
		var addr;
		if ( error ) {
			throw error;
		}
		debug( 'Server started.' );
		server = _server;
		server.on( 'connection', onConnection );
		server.once( 'close', onClose );
		if ( opts.open ) {
			addr = server.address();
			openURL( 'http://'+addr.address+':'+addr.port );
		}
		clbk( null, server );
	}
Callback invoked upon creating a server.

@private
@param {(Error|null)} error - error object
@param {Server} _server - server instance
@throws {Error} error encountered when starting server