hgclient: look for an open server before closing it in ResponseError block At least some of the codepaths that can throw a ResponseError close the server first (the one in _readchannel does so in order to detect server startup failures, for example), so we have to verify we have a server to close before doing so, otherwise we can lose the ResponseError and the user sees an AttributeError when we try to use .close() on a NoneType.