Doesn't reject invalid quantiles

[carbocation]

Due to PBKAC, I was inputting quantiles such as []float64{1, 10, 50, 90, 99} rather than []float64{0.01, 0.1, 0.5, 0.9, 0.99}. The program was crashing due to index out of range errors triggered in quantile.(*Stream).Query(), which seems less obvious to me than to receive an error that indicates that my requested quantiles were invalid. There aren't errors returned by NewTargeted() or by Query(), so the changes that seem obvious to me would be API-breaking.