fix return type

Interestingly, the RPC call was perfectly happy filling the
params.BoolResults with the apiserver return of type params.ErrorResults

I expect that is because params.ErrorResults has the same Results field
with []params.ErrorResult instead of []params.BoolResult, and the only
difference between ErrorResult and BoolResult is that BoolResult has an
additional field for boolean results, and the apiserver code only
populated the Error field which was common between the two.