Change SpawnedSubsystem type to log subsystem errors (#1878)

* Change SpawnedSubsystem type to log subsystem errors

* Remove clone

polkadot/node/core/chain-api/src/lib.rs

@@ -67,7 +67,6 @@ impl<Client, Context> Subsystem<Context> for ChainApiSubsystem<Client> where
 	fn start(self, ctx: Context) -> SpawnedSubsystem {
 		let future = run(ctx, self)
 			.map_err(|e| SubsystemError::with_origin("chain-api", e))
-			.map(|_| ())
 			.boxed();
 		SpawnedSubsystem {
 			future,