nth-prime: Use object instead of bool for invalid value (#969)
nth-prime: Use object instead of bool for invalid value (#969)

* Use object instead of bool for invalid value in nth-prime

exercism/x-common,jmluy/x-common,ErikSchierboom/x-common,petertseng/x-common,exercism/x-common,ErikSchierboom/x-common,petertseng/x-common,rpottsoh/x-common,rpottsoh/x-common,jmluy/x-common,jmluy/x-common