eth/ethconfig: improve error message if TTD missing (#30807)

This updates the message you get when trying to initialize Geth with
genesis.json that doesn't have `terminalTotalDifficulty`. The previous
message was a bit obscure, I had to check the code to find out what the
problem was.

eth/ethconfig/config.go

@@ -32,6 +32,7 @@ import (
 	"github.com/ethereum/go-ethereum/eth/downloader"
 	"github.com/ethereum/go-ethereum/eth/gasprice"
 	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/miner"
 	"github.com/ethereum/go-ethereum/params"
 )
@@ -160,7 +161,8 @@ type Config struct {
 // only exist on already merged networks.
 func CreateConsensusEngine(config *params.ChainConfig, db ethdb.Database) (consensus.Engine, error) {
 	if config.TerminalTotalDifficulty == nil {
-		return nil, fmt.Errorf("only PoS networks are supported, please transition old ones with Geth v1.13.x")
+		log.Error("Geth only supports PoS networks. Please transition legacy networks using Geth v1.13.x.")
+		return nil, fmt.Errorf("'terminalTotalDifficulty' is not set in genesis block")
 	}
 	// Wrap previously supported consensus engines into their post-merge counterpart
 	if config.Clique != nil {