let-or-reply misleading error message

[tank157]

Not sure if there's really anything to do about this, but I spent a little while tracking down an error with my use of let-or-reply and the error message was leading me wrong. I'd missed the need to pass ctx in, so my bindings became ctx, and my exprs became bindings, and i was getting the not divisible by 3 error, which didn't look like a problem. So it's not really critical, I was doing the wrong thing, but it would be nice if it could help detect something like this.