`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
void Serialize(Stream& s) const
    {
        char fSpent = false;
        mapValue_t mapValueCopy = mapValue;
        mapValueCopy["fromaccount"] = "";
        WriteOrderPos(nOrderPos, mapValueCopy);
        if (nTimeSmart) {
            mapValueCopy["timesmart"] = strprintf("%u", nTimeSmart);
        }
        s << static_cast<const CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s << vUnused << mapValueCopy << vOrderForm << fTimeReceivedIsTxTime << nTimeReceived << fFromMe << fSpent;
    }
    template<typename Stream>
    void Unserialize(Stream& s)
    {
        Init(nullptr);
        char fSpent;
        s >> static_cast<CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s >> vUnused >> mapValue >> vOrderForm >> fTimeReceivedIsTxTime >> nTimeReceived >> fFromMe >> fSpent;
        ReadOrderPos(nOrderPos, mapValue);
        nTimeSmart = mapValue.count("timesmart") ? (unsigned int)atoi64(mapValue["timesmart"]) : 0;
        mapValue.erase("fromaccount");
        mapValue.erase("spent");
        mapValue.erase("n");
        mapValue.erase("timesmart");
    }
    
    void MarkDirty()