async function checkGraphvizInstalled(config) {
	const cmd = config.graphVizPath ? path.join(config.graphVizPath, 'gvpr') : 'gvpr';

	try {
		await exec(cmd, ['-V']);
	} catch (err) {
		if (err.code === 'ENOENT') {
			throw new Error(`Graphviz could not be found. Ensure that "gvpr" is in your $PATH. ${err}`);
		} else {
			throw new Error(`Unexpected error when calling Graphviz "${cmd}". ${err}`);
		}
	}
}
Check if Graphviz is installed on the system.
@param  {Object} config
@return {Promise}