Change SpawnedSubsystem type to log subsystem errors (#1878)

* Change SpawnedSubsystem type to log subsystem errors

* Remove clone

polkadot/node/core/candidate-validation/src/lib.rs

@@ -121,7 +121,6 @@ impl<S, C> Subsystem<C> for CandidateValidationSubsystem<S> where
 	fn start(self, ctx: C) -> SpawnedSubsystem {
 		let future = run(ctx, self.spawn, self.metrics)
 			.map_err(|e| SubsystemError::with_origin("candidate-validation", e))
-			.map(|_| ())
 			.boxed();
 		SpawnedSubsystem {
 			name: "candidate-validation-subsystem",