function exitIfNoReleaseDir(releaseDir, done) {
  // Check that release directory exists.
  if (!fs.existsSync(releaseDir)) {
    console.error(
      `No release directory ${releaseDir} exists. ` +
        `Did you run 'npm run publish:prepare'?`,
    );
    done();
    process.exit(1);
  }
}
Exit early if the release directory does not exist.
@param {string} releaseDir release directory to check
@param {Function} done Gulp callback.