I think that if a reorg fails due to an error while disconnecting blocks, the node should shut down.

My thought is that if we are trying to switch to a more work chain and something goes wrong before we can even try to validate that more-work chain, then we have a problem that is not related to the chain we're trying to switch to but instead related to our own hardware or software; shutting down and having the user deal with the issue is better than continuing on, when we know there's a more work chain that we're unable to try validating.

This has recently come up on testnet, where unpatched nodes are unable to reorg from the invalid chain containing the duplicate-input-in-a-transaction block, but rather than crash or shutdown they are continuing on the invalid chain, with messages like this in the debug log:

```
ERROR: DisconnectTip(): DisconnectBlock 00000000210004840364b52bc5e455d888f164e4264a4fec06a514b67e9d5722 failed
```
if (!tx.vout[o].scriptPubKey.IsUnspendable()) {
                COutPoint out(hash, o);
                Coin coin;
                bool is_spent = view.SpendCoin(out, &coin);
                if (!is_spent || tx.vout[o] != coin.out || pindex->nHeight != coin.nHeight || is_coinbase != coin.fCoinBase) {
                    fClean = false; 
                }
            }
        }
        
        if (i > 0) { 
            CTxUndo &txundo = blockUndo.vtxundo[i-1];
            if (txundo.vprevout.size() != tx.vin.size()) {
                error("DisconnectBlock(): transaction and undo data inconsistent");
                return DISCONNECT_FAILED;
            }
            for (unsigned int j = tx.vin.size(); j-- > 0;) {
                const COutPoint &out = tx.vin[j].prevout;
                int res = ApplyTxInUndo(std::move(txundo.vprevout[j]), view, out);
                if (res == DISCONNECT_FAILED) return DISCONNECT_FAILED;
                fClean = fClean && res != DISCONNECT_UNCLEAN;
            }
            
        }
    }
    
    view.SetBestBlock(pindex->pprev->GetBlockHash());
    return fClean ? DISCONNECT_OK : DISCONNECT_UNCLEAN;
}
void static FlushBlockFile(bool fFinalize = false)