Send statements to own backing group first (#2927)

* Factor out runtime module into utils.

* First fatal error design.

* Better error handling infra.

* Error handling cleanup.

* Send to peers of our group first.

* Finish backing group prioritization.

* Little cleanup.

* More cleanup.

* Forgot to checkin error.rs.

* Notes.

* Runtime -> RuntimeInfo

* qed in debug assert.

* PolkaErr -> Fault.

polkadot/node/network/availability-distribution/src/requester/fetch_task/mod.rs

@@ -34,7 +34,7 @@ use polkadot_subsystem::messages::{
 use polkadot_subsystem::{SubsystemContext, jaeger};
 
 use crate::{
-	error::{Error, Result},
+	error::{Fatal, Result},
 	session_cache::{BadValidators, SessionInfo},
 	LOG_TARGET,
 	metrics::{Metrics, SUCCEEDED, FAILED},
@@ -191,7 +191,7 @@ impl FetchTask {
 
 			ctx.spawn("chunk-fetcher", running.run(kill).boxed())
 				.await
-				.map_err(|e| Error::SpawnTask(e))?;
+				.map_err(|e| Fatal::SpawnTask(e))?;
 
 			Ok(FetchTask {
 				live_in,