fix #157: check download tool before use it
fix #157: check download tool before use it

myself659/xunlei-lixian,GeassDB/xunlei-lixian,xieyanhao/xunlei-lixian,wangjun/xunlei-lixian,iambus/xunlei-lixian,sdgdsffdsfff/xunlei-lixian,windygu/xunlei-lixian,wogong/xunlei-lixian,ccagg/xunlei,liujianpc/xunlei-lixian,sndnvaps/xunlei-lixian,davies/xunlei-lixian