The panic message lead people to believe some userland CAM request had caused a problem when in reallity it was for a kernel request (eg the USER bit was cleared). Reword message. Also, improve a couple of comments to reflect that the periph shouldn't be completely torn down before we get here (so the path and sim pointers should be valid, but aren't and the code is designed to be robust enough in the face of that to give a specific panic message).