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)
 }

net/ghttp/ghttp_server_session.go

@@ -186,7 +186,7 @@ func (s *Session) UpdateExpire() {
 				time.Duration(s.server.GetSessionMaxAge())*time.Second,
 			)
 			if err != nil {
-				panic(err)
+				panic("saving session failed:" + err.Error())
 			}
 		}
 		// 其次更新内存TTL