refac: missing user or article is reported with a domain not-found error instead of a raw GORM error, and the API answers 404 for it

internal/domain/user/userDomain.go

@@ -1,9 +1,12 @@
 package user
 
 import (
+	"errors"
 	"time"
 )
 
+var ErrUserNotFound = errors.New("user not found")
+
 type User struct {
 	ID                     uint32    `json:"id"`
 	Username               string    `json:"username"`