replace log.Fatal with cobra.CheckErr

cmd/gcd.go

@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -47,7 +46,7 @@ func gcd(cmd *cobra.Command, args []string) {
 	for i, s := range args {
 		_, ok := w.SetString(s, 10)
 		if !ok {
-			log.Fatal("invalid input " + s)
+			cobra.CheckErr("invalid input " + s)
 		}
 
 		z.GCD(x, y, z, w)