refac: better error handling & moved IsOnline logic to cleaner place

internal/domain/article/articleDomain.go

@@ -2,9 +2,15 @@ package article
 
 import (
 	domain "RenewCMS/internal/domain/image"
+	"errors"
 	"time"
 )
 
+var (
+	ErrArticleNotFound = errors.New("article not found")
+	ErrArticleOffline  = errors.New("article is offline")
+)
+
 type Article struct {
 	ID        uint32          `json:"id"`
 	Title     string          `json:"title"`