I have gotten these 2 before. Here is what I did. Take the sd card out, use the computer to put the SuperSU.zip (Downloaded from XDA) on it. Downloaded VirperOneM8 (Or any other rom will probably work) rom from XDA, put it on sd card, put card back in phone. Then I went back into recovery, hit install go to the sd card, install the supersu, then hit install again and install the rom. (I had nandroid backups of my rom, (I am kind of anal about that, make them every time I change things) So I was back where I wanted to be. Hope this works for you.