VerifyRandao returns proper error when parent state is unavailable

[hyunsooda]

Proposed changes

Make VerifyRandao() return an appropriate error when the parent block state is not found.

Currently, the VerifyRandao() may return a "missing trie node" error when the parent state is not found. Specifically, the error may be triggered after a rewinding by debug_setHead, leading to downloader requests for older blocks lacking the required state. As these blocks undergo verification, they reach VerifyRandao(), but in full mode node the parent states are not stored, thereby throwing the "missing trie node" error.

The parent state is needed to determine the proposer's BLS public key, which is then used to verify the randomReveal field.

If the parent state is not found, VerifyHeader() now returns a consensus.ErrPrunedAncestor error so that BlockChain.InsertChain can take state regeneration path in https://github.com/klaytn/klaytn/blob/v1.12.0/blockchain/blockchain.go#L1992-L2017.

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