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

api/handlers/article/articleHandler.go

@@ -46,7 +46,7 @@ func (h *Handler) GetArticle(w http.ResponseWriter, r *http.Request) {
 	article, err := h.GetUseCase.GetOnlineArticle(uint32(id))
 	if err != nil {
 		if errors.Is(err, domain.ErrArticleNotFound) {
-			http.Error(w, "Article not found", http.StatusNotFound)
+			http.Error(w, apperr.ArticleNotFoundErrorMessage, http.StatusNotFound)
 			return
 		}
 		http.Error(w, "Internal server error", http.StatusInternalServerError)