refactor: improve error message for non-existent files in document handling

--bug=1057925 --user=刘瑞斌 【知识库】知识库快速创建空白文档-添加段落和内容-随后下载源文档-下载文件提示异常 https://www.tapd.cn/62980211/s/1723202

Author: liuruibin

apps/knowledge/serializers/document.py

@@ -563,7 +563,7 @@ def download_source_file(self):
             self.is_valid(raise_exception=True)
             file = QuerySet(File).filter(source_id=self.data.get('document_id')).first()
             if not file:
-                raise AppApiException(500, _('file not exist'))
+                raise AppApiException(500, _('File not exist. Only manually uploaded documents are supported.'))
             return FileSerializer.Operate(data={'id': file.id}).get(with_valid=True)
 
         def one(self, with_valid=False):