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
```
{
    return strprintf("%s%s (code %i)",
        state.GetRejectReason(),
        state.GetDebugMessage().empty() ? "" : ", "+state.GetDebugMessage(),
        state.GetRejectCode());
}
static bool IsCurrentForFeeEstimation() EXCLUSIVE_LOCKS_REQUIRED(cs_main)
{
    AssertLockHeld(cs_main);
    if (IsInitialBlockDownload())
        return false;
    if (chainActive.Tip()->GetBlockTime() < (GetTime() - MAX_FEE_ESTIMATION_TIP_AGE))
        return false;
    if (chainActive.Height() < pindexBestHeader->nHeight - 1)
        return false;
    return true;
}
/* Make mempool consistent after a reorg, by re-adding or recursively erasing
 * disconnected block transactions from the mempool, and also removing any
 * other transactions from the mempool that are no longer valid given the new
 * tip/height.
 *
 * Note: we assume that disconnectpool only contains transactions that are NOT
 * confirmed in the current chain nor already in the mempool (otherwise,
 * in-mempool descendants of such transactions would be removed).
 *
 * Passing fAddToMempool=false will skip trying to add the transactions back,
 * and instead just erase from the mempool as needed.
 */
static void UpdateMempoolForReorg(DisconnectedBlockTransactions &disconnectpool, bool fAddToMempool) EXCLUSIVE_LOCKS_REQUIRED(cs_main)