Improved diagnostics for cases where FindBox is called without a literal

I was pulling my hair out trying to figure out why `rice` wasn't working for me.  It turned out that it was silently ignoring calls to `FindBox` because the argument wasn't a literal.  So I added some output when the verbose flag was set to hint at the problem.  Frankly, I think this should actually be a call to `fmt.Printf` because if somebody is doing this, it is absolutely wrong.  But I wanted to be more conservative about it for now and let you decide.