rpc-author: Downgrade the logging level of some logs (#10483)

* rpc-author: Downgrade the logging level of some logs

Currently these messages are printed with `warn` levels, but they don't bring the node operator
anything. The problem with these log lines is that they are triggered by external users when they
send invalid transactions. This pr "resolves" this by downgrading the log severity to debug, aka not
shown by default.

* FMT

client/rpc/src/author/mod.rs

@@ -21,7 +21,6 @@
 #[cfg(test)]
 mod tests;
 
-use log::warn;
 use std::{convert::TryInto, sync::Arc};
 
 use sp_blockchain::HeaderBackend;
@@ -188,7 +187,7 @@ where
 		let dxt = match TransactionFor::<P>::decode(&mut &xt[..]).map_err(error::Error::from) {
 			Ok(tx) => tx,
 			Err(err) => {
-				warn!("Failed to submit extrinsic: {}", err);
+				log::debug!("Failed to submit extrinsic: {}", err);
 				// reject the subscriber (ignore errors - we don't care if subscriber is no longer
 				// there).
 				let _ = subscriber.reject(err.into());
@@ -211,7 +210,7 @@ where
 			let tx_stream = match submit.await {
 				Ok(s) => s,
 				Err(err) => {
-					warn!("Failed to submit extrinsic: {}", err);
+					log::debug!("Failed to submit extrinsic: {}", err);
 					// reject the subscriber (ignore errors - we don't care if subscriber is no
 					// longer there).
 					let _ = subscriber.reject(err.into());
@@ -222,14 +221,16 @@ where
 			subscriptions.add(subscriber, move |sink| {
 				tx_stream
 					.map(|v| Ok(Ok(v)))
-					.forward(sink.sink_map_err(|e| warn!("Error sending notifications: {:?}", e)))
+					.forward(
+						sink.sink_map_err(|e| log::debug!("Error sending notifications: {:?}", e)),
+					)
 					.map(drop)
 			});
 		};
 
 		let res = self.subscriptions.executor().spawn_obj(future.boxed().into());
 		if res.is_err() {
-			warn!("Error spawning subscription RPC task.");
+			log::warn!("Error spawning subscription RPC task.");
 		}
 	}