func (m *muxClient) addErrorNonBlockingClose(respHandler chan<- Response, err error) {
	m.respMu.Lock()
	defer m.respMu.Unlock()
	if !m.closed {
		m.respErr.Store(&err)
		// Do not block.
		select {
		case respHandler <- Response{Err: err}:
			xioutil.SafeClose(respHandler)
		default:
			go func() {
				respHandler <- Response{Err: err}
				xioutil.SafeClose(respHandler)
			}()
		}