Better error for spurious cache misses. (#4669)

Happened on Kusama for some not yet known reason.

polkadot/node/network/availability-distribution/src/error.rs

@@ -18,6 +18,7 @@
 //! Error handling related code and Error/Result definitions.
 
 use polkadot_node_network_protocol::request_response::outgoing::RequestError;
+use polkadot_primitives::v1::SessionIndex;
 use thiserror::Error;
 
 use futures::channel::oneshot;
@@ -76,8 +77,8 @@ pub enum NonFatal {
 	QueryAvailableDataResponseChannel(#[source] oneshot::Canceled),
 
 	/// We tried accessing a session that was not cached.
-	#[error("Session is not cached.")]
-	NoSuchCachedSession,
+	#[error("Session {missing_session} is not cached, cached sessions: {available_sessions:?}.")]
+	NoSuchCachedSession { available_sessions: Vec<SessionIndex>, missing_session: SessionIndex },
 
 	/// Sending request response failed (Can happen on timeouts for example).
 	#[error("Sending a request's response failed.")]
@@ -120,7 +121,7 @@ pub fn log_error(result: Result<()>, ctx: &'static str) -> std::result::Result<(
 			match error {
 				NonFatal::UnexpectedPoV |
 				NonFatal::InvalidValidatorIndex |
-				NonFatal::NoSuchCachedSession |
+				NonFatal::NoSuchCachedSession { .. } |
 				NonFatal::QueryAvailableDataResponseChannel(_) |
 				NonFatal::QueryChunkResponseChannel(_) =>
 					tracing::warn!(target: LOG_TARGET, error = %error, ctx),