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

test/article_test.go

@@ -55,7 +55,7 @@ func (m *mockArticleRepo) Get(id uint32) (article.Article, error) {
 	}
 	a, ok := m.articles[id]
 	if !ok {
-		return article.Article{}, gorm.ErrRecordNotFound
+		return article.Article{}, article.ErrArticleNotFound
 	}
 	return a, nil
 }
@@ -70,7 +70,7 @@ func (m *mockArticleRepo) GetByName(name string) (article.Article, error) {
 			return a, nil
 		}
 	}
-	return article.Article{}, gorm.ErrRecordNotFound
+	return article.Article{}, article.ErrArticleNotFound
 }
 
 func (m *mockArticleRepo) GetAll() []article.Article {