在庫追加時のエラーを実装

frontend/src/pages/StockPage.tsx

@@ -123,8 +123,21 @@ const StockPage: React.FC = () => {
       if (newStock.newAddition) {
         newStock.stuffId = null;
       }
-      if (isNaN(newStock.amount)) return;
-      if (newStock.price === null || isNaN(newStock.price)) return;
+
+      if (!newStock.stuffId && !newStock.stuffName) {
+        showErrorMessage(GENERAL_ERRORS.INVALID_STUFF_NAME);
+        return;
+      }
+
+      if (isNaN(newStock.amount)) {
+        showErrorMessage(GENERAL_ERRORS.INVALID_AMOUNT);
+        return;
+      }
+
+      if (newStock.price === null || isNaN(newStock.price)) {
+        showErrorMessage(GENERAL_ERRORS.INVALID_PRICE);
+        return;
+      }
 
       if (newStock.buyAmount !== null && isNaN(newStock.buyAmount)) {
         newStock.buyAmount = null;