feat: add tip for exposing headers etags (#725)

Author: weiyie

example/README.md

@@ -30,7 +30,7 @@ access.
 
 ### App setup
 
-Fill in your bucket name  and region in `app.js`: (Note: ensure port 9000 is not used)
+Fill in your bucket name  and region in `main.js`: (Note: ensure port 9000 is not used)
 
 ```js
 var bucket = '<your bucket name>';

lib/common/multipart.js

@@ -239,6 +239,10 @@ proto._uploadPart = async function _uploadPart(name, uploadId, partNo, data, opt
 
   const result = await this.request(params);
 
+  if (!result.res.headers.etag) {
+    throw new Error('Please set the etag of expose-headers in OSS \n https://help.aliyun.com/document_detail/32069.html');
+  }
+
   data.stream = null;
   params.stream = null;
   return {