fatality based errors (#4448)

* seed commit for fatality based errors

* fatality

* first draft of fatality

* cleanup

* differnt approach

* simplify

* first working version for enums, with documentation

* add split

* fix simple split test case

* extend README.md

* update fatality impl

* make tests passed

* apply fatality to first subsystem

* fatality fixes

* use fatality in a subsystem

* fix subsystemg

* fixup proc macro

* fix/test: log::*! do not execute when log handler is missing

* fix spelling

* rename Runtime2 to something sane

* allow nested split with `forward` annotations

* add free license

* enable and fixup all tests

* use external fatality

Makes this more reviewable.

* bump fatality dep

Avoid duplicate expander compilations.

* migrate availability distribution

* more fatality usage

* chore: bump fatality to 0.0.6

* fixup remaining subsystems

* chore: fmt

* make cargo spellcheck happy

* remove single instance of `#[fatal(false)]`

* last quality sweep

* fixup

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

@@ -39,7 +39,7 @@ use polkadot_subsystem::{
 };
 
 use crate::{
-	error::{Fatal, Result},
+	error::{FatalError, Result},
 	metrics::{Metrics, FAILED, SUCCEEDED},
 	requester::session_cache::{BadValidators, SessionInfo},
 	LOG_TARGET,
@@ -185,7 +185,7 @@ impl FetchTask {
 			let (handle, kill) = oneshot::channel();
 
 			ctx.spawn("chunk-fetcher", running.run(kill).boxed())
-				.map_err(|e| Fatal::SpawnTask(e))?;
+				.map_err(|e| FatalError::SpawnTask(e))?;
 
 			Ok(FetchTask { live_in, state: FetchedState::Started(handle) })
 		} else {