eth, les: add error when accessing missing block state (#18346)

This change makes getBalance, getCode, getStorageAt, getProof,
call, getTransactionCount return an error if the block number in
the request doesn't exist. getHeaderByNumber still returns null
for missing headers.

eth/api_backend.go

@@ -18,6 +18,7 @@ package eth
 
 import (
 	"context"
+	"errors"
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/accounts"
@@ -95,9 +96,12 @@ func (b *EthAPIBackend) StateAndHeaderByNumber(ctx context.Context, blockNr rpc.
 	}
 	// Otherwise resolve the block number and return its state
 	header, err := b.HeaderByNumber(ctx, blockNr)
-	if header == nil || err != nil {
+	if err != nil {
 		return nil, nil, err
 	}
+	if header == nil {
+		return nil, nil, errors.New("header not found")
+	}
 	stateDb, err := b.eth.BlockChain().StateAt(header.Root)
 	return stateDb, header, err
 }