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
```
#include <script/sigcache.h>
#include <script/standard.h>
#include <shutdown.h>
#include <timedata.h>
#include <tinyformat.h>
#include <txdb.h>
#include <txmempool.h>
#include <ui_interface.h>
#include <undo.h>
#include <util/system.h>
#include <util/moneystr.h>
#include <util/strencodings.h>
#include <validationinterface.h>
#include <warnings.h>
#include <future>
#include <sstream>
#include <boost/algorithm/string/replace.hpp>
#include <boost/thread.hpp>
#if defined(NDEBUG)
# error "Bitcoin cannot be compiled without assertions."
#endif
#define MICRO 0.000001
#define MILLI 0.001
/**
 * Global state
 */
namespace {
    struct CBlockIndexWorkComparator
    {
        bool operator()(const CBlockIndex *pa, const CBlockIndex *pb) const {