function decode_param(val) {
  if (typeof val !== 'string' || val.length === 0) {
    return val;
  }

  try {
    return decodeURIComponent(val);
  } catch (err) {
    if (err instanceof URIError) {
      err.message = 'Failed to decode param \'' + val + '\'';
      err.status = err.statusCode = 400;
    }

    throw err;
  }
}
Decode param value.

@param {string} val
@return {string}
@private