kvserver: handle context cancellation when receiving snapshots

In a3fd4fb, context errors are now propagated back up the stack
when receiving snapshots. However, a `maybeFatalOnRaftReadyErr`
assertion further up the stack was not prepared to handle these new
errors and crash the node.

This patch explicitly propagates the context cancellation errors on the
snapshot receival code path, returning it up the stack. It does not
modify `maybeFatalOnRaftReadyErr` to allow context cancellation, since
the main Raft `processReady` path appears to use a background context
and improved context handling here likely requires more care.

Release note: None

pkg/kv/kvserver/store_raft.go

@@ -338,14 +338,17 @@ func (s *Store) processRaftSnapshotRequest(
 		var expl string
 		var err error
 		stats, expl, err = r.handleRaftReadyRaftMuLocked(ctx, inSnap)
+		if errors.IsAny(err, context.Canceled, context.DeadlineExceeded) {
+			return roachpb.NewError(err)
+		}
+		maybeFatalOnRaftReadyErr(ctx, expl, err)
 		if !stats.snap.applied {
 			// This line would be hit if a snapshot was sent when it isn't necessary
 			// (i.e. follower was able to catch up via the log in the interim) or when
 			// multiple snapshots raced (as is possible when raft leadership changes
 			// and both the old and new leaders send snapshots).
 			log.Infof(ctx, "ignored stale snapshot at index %d", snapHeader.RaftMessageRequest.Message.Snapshot.Metadata.Index)
 		}
-		maybeFatalOnRaftReadyErr(ctx, expl, err)
 		return nil
 	})
 }