cryptonote_protocol: warn when the last connection goes

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -2366,6 +2366,8 @@ skip:
     {
       MINFO("Target height decreasing from " << previous_target << " to " << target);
       m_core.set_target_blockchain_height(target);
+      if (target == 0 && context.m_state > cryptonote_connection_context::state_before_handshake && !m_stopping)
+        MCWARNING("global", "monerod is now disconnected from the network");
     }
 
     m_block_queue.flush_spans(context.m_connection_id, false);