It's possible to call verify when no documents have been cached

[danluu]

Query,TermPos,TruePositives,FalsePositives,FalseNegatives,FalseRate
from,,0,2838699,0,1
also,1,0,2018632,0,1
which,2,0,1961024,0,1
has,3,0,1776560,0,1
first,4,0,1705426,0,1

[danluu]

I think this is user error on my part (using load instead of cache), but it sounds like I'm not the only person to do this. We could prevent this by CHECK failing when we call verify without cache every having been called.