Fix nil Error in function reportBlock

[gzliudan]

Proposed changes

This PR fix a bug in function reportBlock as reported in post and issue #369:

Error: <nil>

The Error is always nil from EngineV2. This bug is caused by the function reportBlock:

func (bc *BlockChain) reportBlock(block *types.Block, receipts types.Receipts, err error) {
	bc.addBadBlock(block)

	// V2 specific logs
	config, _ := json.Marshal(bc.chainConfig)

	var roundNumber = types.Round(0)
	engine, ok := bc.Engine().(*XDPoS.XDPoS)
	if ok {
		roundNumber, err = engine.EngineV2.GetRoundNumber(block.Header())
	}

	var receiptString string
	for _, receipt := range receipts {
		receiptString += fmt.Sprintf("\t%v\n", receipt)
	}
	log.Error(fmt.Sprintf(`
########## BAD BLOCK #########
Chain config: %v

Number: %v
Hash: 0x%x
%v

Round: %v
Error: %v
##############################
`, string(config), block.Number(), block.Hash(), receiptString, roundNumber, err))
}

In above codes, the parameter err is reassigned to value nil by the line:

roundNumber, err = engine.EngineV2.GetRoundNumber(block.Header())

after switch to EngineV2.

Types of changes

What types of changes does your code introduce to XDC network?
Put an ✅ in the boxes that apply

• [✅] Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Regular KTLO or any of the maintaince work. e.g code style
• CICD Improvement

Impacted Components

Which part of the codebase this PR will touch base on,

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• Consensus
• Account
• [✅] Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

Put an ✅ in the boxes once you have confirmed below actions (or provide reasons on not doing so) that

• [✅] This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A