code: function findExecutable(opt_exe) {
    var exe = opt_exe || io.findInPath(PHANTOMJS_EXE, true);
    if (!exe) {
      throw Error(
          'The PhantomJS executable could not be found on the current PATH. ' +
          'Please download the latest version from ' +
          'http://phantomjs.org/download.html and ensure it can be found on ' +
          'your PATH. For more information, see ' +
          'https://github.com/ariya/phantomjs/wiki');
    }
    if (!fs.existsSync(exe)) {
      throw Error('File does not exist: ' + exe);
    }
    return exe;
  }
docs: Finds the PhantomJS executable.
  @param {string=} opt_exe Path to the executable to use.
  @return {string} The located executable.
  @throws {Error} If the executable cannot be found on the PATH, or if the
  provided executable path does not exist.