function _wrap(cb) {
        return function (err) {
            var args = Array.prototype.slice.call(arguments);
            args[0] = _mapError(args[0]);
            cb.apply(null, args);
        };
    }
Convert a callback to one that transforms its first parameter from an
appshell error code to a FileSystemError string.

@param {function(?number)} cb A callback that expects an appshell error code
@return {function(?string)} A callback that expects a FileSystemError string
@private