Add error messages to be displayed in the modal

packages/app/public/static/locales/en_US/translation.json

@@ -389,7 +389,7 @@
     "notfound_or_forbidden": "Original page is not found or forbidden.",
     "already_exists": "Page with the path already exists.",
     "outdated": "Page is updated someone and now outdated.",
-    "user_not_admin": "Only admin user can delete"
+    "single_deletion_empty_pages": "Empty pages cannot be single deleted"
   },
   "page_history": {
     "revision_list": "Revision list",

packages/app/public/static/locales/ja_JP/translation.json

@@ -380,7 +380,8 @@
     "notfound_or_forbidden": "元のページが見つからないか、アクセス権がありません。",
     "already_exists": "そのパスを持つページは既に存在しています。",
     "outdated": "ページが他のユーザーによって更新されました。",
-    "user_not_admin": "権限のあるユーザーのみが削除できます"
+    "user_not_admin": "権限のあるユーザーのみが削除できます",
+    "single_deletion_empty_pages": "空ページの単体削除はできません"
   },
   "page_history": {
     "revision_list": "更新履歴",

packages/app/public/static/locales/zh_CN/translation.json

@@ -362,7 +362,8 @@
 		"notfound_or_forbidden": "未找到或禁止原始页。",
 		"already_exists": "具有该路径的页面已存在",
 		"outdated": "页面已被某人更新，现在已过时。",
-		"user_not_admin": "仅管理员用户可以删除"
+		"user_not_admin": "仅管理员用户可以删除",
+    "single_deletion_empty_pages": "空的页面不能被单一删除"
   },
   "page_history": {
     "revision_list": "修订清单",

packages/app/src/components/PageManagement/ApiErrorMessage.jsx

@@ -1,7 +1,7 @@
 import React from 'react';
 
-import PropTypes from 'prop-types';
 import { useTranslation } from 'next-i18next';
+import PropTypes from 'prop-types';
 
 const ApiErrorMessage = (props) => {
   const { t } = useTranslation();
@@ -43,6 +43,10 @@ const ApiErrorMessage = (props) => {
         return (
           <strong><i className="icon-fw icon-ban"></i> Invalid path</strong>
         );
+      case 'single_deletion_empty_pages':
+        return (
+          <strong><i className="icon-fw icon-ban"></i>{ t('page_api_error.single_deletion_empty_pages') }</strong>
+        );
       default:
         return (
           <strong><i className="icon-fw icon-ban"></i> Unknown error occured</strong>

packages/app/src/server/routes/page.js

@@ -1282,7 +1282,7 @@ module.exports = function(crowi, app) {
     }
 
     if (page.isEmpty && !isRecursively) {
-      return res.json(ApiResponse.error('Empty pages cannot be single deleted'));
+      return res.json(ApiResponse.error('Empty pages cannot be single deleted', 'single_deletion_empty_pages'));
     }
 
     let creator;