`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.
{
    {
        LOCK(cs_wallet);
        for (std::pair<const uint256, CWalletTx>& item : mapWallet)
            item.second.MarkDirty();
    }
}
bool CWallet::MarkReplaced(const uint256& originalHash, const uint256& newHash)
{
    LOCK(cs_wallet);
    auto mi = mapWallet.find(originalHash);
    
    assert(mi != mapWallet.end());
    CWalletTx& wtx = (*mi).second;
    
    assert(wtx.mapValue.count("replaced_by_txid") == 0);
    wtx.mapValue["replaced_by_txid"] = newHash.ToString();
    WalletBatch batch(*database, "r+");
    bool success = true;
    if (!batch.WriteTx(wtx)) {
        WalletLogPrintf("%s: Updating batch tx %s failed\n", __func__, wtx.GetHash().ToString());
        success = false;
    }
    NotifyTransactionChanged(this, originalHash, CT_UPDATED);
    return success;
}
bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose)