fix connection not release bug (#353)

* fix connection not release bug

* Address Nicole00's comment as agreed in review

---------

Co-authored-by: Wey Gu <weyl.gu@gmail.com>

Author: ghjhhyuyuy

nebula3/gclient/net/Connection.py

@@ -171,6 +171,7 @@ def authenticate(self, user_name, password):
         try:
             resp = self._connection.authenticate(user_name, password)
             if resp.error_code != ErrorCode.SUCCEEDED:
+                self._connection.is_used = False
                 raise AuthFailedException(resp.error_msg)
             return AuthResult(
                 resp.session_id, resp.time_zone_offset_seconds, resp.time_zone_name