Improve error message for missing Celo1 blocks

eth/backend.go

@@ -258,8 +258,10 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 	// Make sure Cel2 full sync node has migrated data from Celo1
 	currentBlock := eth.blockchain.CurrentBlock()
 	if config.SyncMode == downloader.FullSync && chainConfig.Cel2Time != nil && !chainConfig.IsCel2(currentBlock.Number.Uint64()) {
-		log.Error("Migrated data for Cel2 is missing, please ensure the migrated data from Celo1 can be properly loaded before starting the blockchain",
-			"current number", currentBlock.Number.Uint64(), "current hash", currentBlock.Hash())
+		log.Error(fmt.Sprintf(
+			"Chaindata is missing blocks, Cel2 fork is at block %d, but the head block is %d. Pre Cel2 blocks must be migrated from"+
+				" Celo L1 chaindata, Celo L2 cannot generate pre Cel2 blocks. See migration instructions here - https://docs.celo.org/cel2/l2-operator-guide",
+			*chainConfig.Cel2Time, currentBlock.Number.Uint64()))
 		return nil, fmt.Errorf("missing migrated data")
 	}
 	if chainConfig := eth.blockchain.Config(); chainConfig.Optimism != nil { // config.Genesis.Config.ChainID cannot be used because it's based on CLI flags only, thus default to mainnet L1