Log when a channel is closed on startup due to stale ChannelManager

This is one of the riskiest parts of our API from the perspective
of accidental force-closes - if users delay persisting the
ChannelManager much at all after a ChannelMonitor we may hit a
force-close after restart.

The fact that we don't log at all when this happens is criminal.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4960,6 +4960,9 @@ impl<'a, Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref>
 						channel.get_cur_counterparty_commitment_transaction_number() > monitor.get_cur_counterparty_commitment_number() ||
 						channel.get_latest_monitor_update_id() < monitor.get_latest_update_id() {
 					// But if the channel is behind of the monitor, close the channel:
+					log_error!(args.logger, "A ChannelManager is stale compared to the current ChannelMonitor! The channel will be closed.");
+					log_error!(args.logger, " The ChannelMonitor for channel {} is at update_id {} but the ChannelManager is at update_id {}.",
+						log_bytes!(channel.channel_id()), monitor.get_latest_update_id(), channel.get_latest_monitor_update_id());
 					let (_, mut new_failed_htlcs) = channel.force_shutdown(true);
 					failed_htlcs.append(&mut new_failed_htlcs);
 					monitor.broadcast_latest_holder_commitment_txn(&args.tx_broadcaster, &args.logger);