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
```
{
    LOCK(cs_LastBlockFile);
    CDiskBlockPos posOld(nLastBlockFile, 0);
    bool status = true;
    FILE *fileOld = OpenBlockFile(posOld);
    if (fileOld) {
        if (fFinalize)
            status &= TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nSize);
        status &= FileCommit(fileOld);
        fclose(fileOld);
    }
    fileOld = OpenUndoFile(posOld);
    if (fileOld) {
        if (fFinalize)
            status &= TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nUndoSize);
        status &= FileCommit(fileOld);
        fclose(fileOld);
    }
    if (!status) {
        AbortNode("Flushing block file to disk failed. This is likely the result of an I/O error.");
    }
}
static bool FindUndoPos(CValidationState &state, int nFile, CDiskBlockPos &pos, unsigned int nAddSize);
static bool WriteUndoDataForBlock(const CBlockUndo& blockundo, CValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)