Bridge: ignore client errors when calling recently added *_free_headers_interval methods (paritytech#4350)

see paritytech/parity-bridges-common#2974 : we
still need to support unupgraded chains (BHK and BHP) in relay

We may need to revert this change when all chains are upgraded

Author: svyatonik

bridges/relays/lib-substrate-relay/src/finality/target.rs

@@ -107,13 +107,24 @@ impl<P: SubstrateFinalitySyncPipeline> TargetClient<FinalitySyncPipelineAdapter<
 	async fn free_source_headers_interval(
 		&self,
 	) -> Result<Option<BlockNumberOf<P::SourceChain>>, Self::Error> {
-		self.client
+		Ok(self
+			.client
 			.typed_state_call(
 				P::SourceChain::FREE_HEADERS_INTERVAL_METHOD.into(),
 				(),
 				Some(self.client.best_header().await?.hash()),
 			)
 			.await
+			.unwrap_or_else(|e| {
+				log::info!(
+					target: "bridge",
+					"Call of {} at {} has failed with an error: {:?}. Treating as `None`",
+					P::SourceChain::FREE_HEADERS_INTERVAL_METHOD,
+					P::TargetChain::NAME,
+					e,
+				);
+				None
+			}))
 	}
 
 	async fn submit_finality_proof(

bridges/relays/lib-substrate-relay/src/parachains/target.rs

@@ -122,9 +122,20 @@ where
 	async fn free_source_relay_headers_interval(
 		&self,
 	) -> Result<Option<BlockNumberOf<P::SourceRelayChain>>, Self::Error> {
-		self.target_client
+		Ok(self
+			.target_client
 			.typed_state_call(P::SourceRelayChain::FREE_HEADERS_INTERVAL_METHOD.into(), (), None)
 			.await
+			.unwrap_or_else(|e| {
+				log::info!(
+					target: "bridge",
+					"Call of {} at {} has failed with an error: {:?}. Treating as `None`",
+					P::SourceRelayChain::FREE_HEADERS_INTERVAL_METHOD,
+					P::TargetChain::NAME,
+					e,
+				);
+				None
+			}))
 	}
 
 	async fn parachain_head(