function onBundle( error, bundle ) {
		if ( error ) {
			debug( 'Encountered an error when generating bundle: %s', error.message );
			return cb( error );
		}
		if ( bundle ) {
			return cb( null, bundle );
		}
		cb( null, true );
	}
Callback invoked upon creating a bundle.

@private
@param {(Error|null)}  error - error object
@param {Buffer} bundle - bundle
@returns {void}