code: function getBinPath(cmd, fn) {
    which(cmd, function(err, bin) {
      if (err) {
        return fn(err);
      }
  
      fs.exists(bin, function(exists) {
        if (!exists) {
          return fn(new Error(format(
            'Expected file for `%s` does not exist at `%s`',
            cmd, bin)));
        }
        fn(null, bin);
      });
    });
  }
docs: Gets the absolute path for a `cmd`.
  @param {String} cmd - e.g. `codesign`.
  @param {Function} fn - Callback which receives `(err, binPath)`.
  @return {void}