pre-checking: Reject failed PVFs (#6492)

* pre-checking: Reject failed PVFs

* paras: immediately reject any PVF that cannot reach a supermajority

* Make the `quorum` reject condition a bit more clear semantically

* Add comment

* Update implementer's guide

* Update a link

Not related to the rest of the PR, but I randomly noticed and fixed this.

* Update runtime/parachains/src/paras/tests.rs

Co-authored-by: s0me0ne-unkn0wn <48632512+s0me0ne-unkn0wn@users.noreply.github.com>

* Remove unneeded loop

* Log PVF retries using `info!`

* Change retry logs to `warn!` and add preparation failure log

* Log PVF execution failure

* Clarify why we reject failed PVFs

* Fix PVF reject runtime benchmarks

Co-authored-by: s0me0ne-unkn0wn <48632512+s0me0ne-unkn0wn@users.noreply.github.com>

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

@@ -295,8 +295,7 @@ where
 			_ => {
 				// The reasoning why this is "failed" and not invalid is because we assume that
 				// during pre-checking voting the relay-chain will pin the code. In case the code
-				// actually is not there, we issue failed since this looks more like a bug. This
-				// leads to us abstaining.
+				// actually is not there, we issue failed since this looks more like a bug.
 				gum::warn!(
 					target: LOG_TARGET,
 					?relay_parent,
@@ -635,7 +634,7 @@ trait ValidationBackend {
 			// Wait a brief delay before retrying.
 			futures_timer::Delay::new(PVF_EXECUTION_RETRY_DELAY).await;
 
-			gum::debug!(
+			gum::warn!(
 				target: LOG_TARGET,
 				?pvf,
 				"Re-trying failed candidate validation due to AmbiguousWorkerDeath."