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/builder.go

@@ -136,7 +136,7 @@ func (b *UpdateBuilder) Finalize(storage Chonk) ([]AUM, error) {
 				needCheckpoint = false
 				break
 			}
-			return nil, fmt.Errorf("reading AUM: %v", err)
+			return nil, fmt.Errorf("reading AUM (%v): %v", cursor, err)
 		}
 
 		if aum.MessageKind == AUMCheckpoint {