improve error text for session saving error

.example/database/gkvdb/main1.go

@@ -2,16 +2,18 @@ package main
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/gogf/gf/database/gkvdb"
 )
 
 func main() {
 	key := []byte("key")
-	//value := []byte("value")
+	value := []byte("value")
 
 	db := gkvdb.Instance()
 	db.SetPath("/tmp/gkvdb/test")
-	//db.Set(key, value)
+	db.Set(key, value)
 	fmt.Println(db.Get(key))
+	time.Sleep(time.Hour)
 }