add error code

README.md

 uploadSdk.onUpdateProgress( progress => {
       console.log(progress)
 })
+```
+
+### 返回的 error 在 catch里面捕捉
+```
+IMAGE_TYPE_ERROR: 101,
+VIDOE_TYPE_ERROR: 102,
+UPLOAD_SUCCESS_CODE: 200,
+UPLOAD_FAILED_CODE: 400
 ```