function showHelp(errorMessage = '') {
  if (errorMessage) {
    console.error(errorMessage);
  }

  console.log(`Auto generates _test.html files for Closure unit tests.

      Usage:
        generate_test_html [OPTIONS]... PATH...

      Paths:
        list of paths to directories or _test.js files. For directories a
        _test.html file will be generated for each _test.js file in the
        directory.

      Options:
        --base
          Path to base.js file.
        --dep_file:
          Optional path to a deps file to use for all tests.
        --recursive
          generate _test.html for each _test.js in each directory recursively.
          Defaults to false.
        --output
          output file path, should be named _test.html. Only valid if a single
          _test.js file is specified for PATH.
        --overwrite
          overwrite any existing _test.html files. Defaults to true.`);
  process.exit(errorMessage ? 1 : 0);
}
Shows script help.
@param {string=} errorMessage