Handle etcd connection error

uncloud/common/etcd_wrapper.py

@@ -65,6 +65,8 @@ class Etcd3Wrapper:
             event_iterator = self.client.get_prefix(*args, **kwargs)
             for e in event_iterator:
                 yield EtcdEntry(*e[::-1], value_in_json=value_in_json)
+        except etcd3.exceptions.ConnectionFailedError as e:
+            raise UncloudException("Cannot connect to etcd: {}".format(e))
         except Exception as err:
             if raise_exception:
                 raise Exception('Exception in etcd_wrapper.get_prefix') from err