Add comment for skipping cleanup due to read error

Author: G8XSU

lightning/src/util/persist.rs

@@ -694,6 +694,7 @@ where
 
 				if let chain::ChannelMonitorUpdateStatus::Completed = monitor_update_status {
 					let cleanup_range = if monitor.get_latest_update_id() == CLOSED_CHANNEL_UPDATE_ID {
+						// If there is an error while reading old monitor, we skip clean up.
 						maybe_old_monitor.map(|(_, ref old_monitor)| {
 							let start = old_monitor.get_latest_update_id();
 							// We never persist an update with update_id = CLOSED_CHANNEL_UPDATE_ID