Small tweaks to staking miner (#3460)

* Fix miner

* fix miner

* Update utils/staking-miner/src/monitor.rs

* Fix width

polkadot/utils/staking-miner/src/monitor.rs

@@ -122,10 +122,21 @@ macro_rules! monitor_cmd_for { ($runtime:tt) => { paste::paste! {
 
 			let mut tx_subscription: Subscription<
 				TransactionStatus<<Block as BlockT>::Hash, <Block as BlockT>::Hash>
-			> = client
+			> = match client
 				.subscribe(&"author_submitAndWatchExtrinsic", params! { bytes }, "author_unwatchExtrinsic")
 				.await
-				.unwrap();
+			{
+				Ok(sub) => sub,
+				Err(why) => {
+				// This usually happens when we've been busy with mining for a few blocks, and now we're receiving the
+				// subscriptions of blocks in which we were busy. In these blocks, we still don't have a solution, so we
+				// re-compute a new solution and submit it with an outdated `Nonce`, which yields most often `Stale`
+				// error. NOTE: to improve this overall, and to be able to introduce an array of other fancy features,
+				// we should make this multi-threaded and do the computation outside of this callback.
+					log::warn!(target: LOG_TARGET, "failing to submit a transaction {:?}. continuing...", why);
+					continue
+				}
+			};
 
 			let _success = while let Some(status_update) = tx_subscription.next().await? {
 				log::trace!(target: LOG_TARGET, "status update {:?}", status_update);