add try/catch to json parse func

rest_client认为每个返回都可解为json,在处理alpaca的撤单操作时报错

vnpy/api/rest/rest_client.py

@@ -268,9 +268,12 @@ class RestClient(object):
             request.response = response
             status_code = response.status_code
             if status_code // 100 == 2:  # 2xx都算成功，尽管交易所都用200
+                try:
                     jsonBody = response.json()
                     request.callback(jsonBody, request)
                     request.status = RequestStatus.success
+                except:
+                    print("not json body from request, check body: ",response.text)
             else:
                 request.status = RequestStatus.failed