ipn, tka: improve the logging around TKA sync and AUM errors

*   When we do the TKA sync, log whether TKA is enabled and whether
    we want it to be enabled. This would help us see if a node is
    making bootstrap errors.

*   When we fail to look up an AUM locally, log the ID of the AUM
    rather than a generic "file does not exist" error.

    These AUM IDs are cryptographic hashes of the TKA state, which
    itself just contains public keys and signatures. These IDs aren't
    sensitive and logging them is safe.

Signed-off-by: Alex Chan <alexc@tailscale.com>

Updates https://github.com/tailscale/corp/issues/33594

tka/tka.go

@@ -102,14 +102,14 @@ func computeChainCandidates(storage Chonk, lastKnownOldest *AUMHash, maxIter int
 
 		iterAgain = false
 		for j := range candidates {
-			parent, hasParent := candidates[j].Oldest.Parent()
+			parentHash, hasParent := candidates[j].Oldest.Parent()
 			if hasParent {
-				parent, err := storage.AUM(parent)
+				parent, err := storage.AUM(parentHash)
 				if err != nil {
 					if err == os.ErrNotExist {
 						continue
 					}
-					return nil, fmt.Errorf("reading parent: %v", err)
+					return nil, fmt.Errorf("reading parent %s: %v", parentHash, err)
 				}
 				candidates[j].Oldest = parent
 				if lastKnownOldest != nil && *lastKnownOldest == parent.Hash() {
@@ -210,7 +210,7 @@ func fastForwardWithAdvancer(
 	}
 	nextAUM, err := storage.AUM(*startState.LastAUMHash)
 	if err != nil {
-		return AUM{}, State{}, fmt.Errorf("reading next: %v", err)
+		return AUM{}, State{}, fmt.Errorf("reading next (%v): %v", *startState.LastAUMHash, err)
 	}
 
 	curs := nextAUM
@@ -297,7 +297,7 @@ func computeStateAt(storage Chonk, maxIter int, wantHash AUMHash) (State, error)
 		// If we got here, the current state is dependent on the previous.
 		// Keep iterating backwards till thats not the case.
 		if curs, err = storage.AUM(parent); err != nil {
-			return State{}, fmt.Errorf("reading parent: %v", err)
+			return State{}, fmt.Errorf("reading parent (%v): %v", parent, err)
 		}
 	}