`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.
auto locked_chain = pwallet->chain().lock();
        LOCK(pwallet->cs_wallet);
        const CWallet::TxItems & txOrdered = pwallet->wtxOrdered;
        
        for (CWallet::TxItems::const_reverse_iterator it = txOrdered.rbegin(); it != txOrdered.rend(); ++it)
        {
            CWalletTx *const pwtx = (*it).second;
            ListTransactions(*locked_chain, pwallet, *pwtx, 0, true, ret, filter);
            if ((int)ret.size() >= (nCount+nFrom)) break;
        }
    }
    
    if (nFrom > (int)ret.size())
        nFrom = ret.size();
    if ((nFrom + nCount) > (int)ret.size())
        nCount = ret.size() - nFrom;
    std::vector<UniValue> arrTmp = ret.getValues();
    std::vector<UniValue>::iterator first = arrTmp.begin();
    std::advance(first, nFrom);
    std::vector<UniValue>::iterator last = arrTmp.begin();
    std::advance(last, nFrom+nCount);
    if (last != arrTmp.end()) arrTmp.erase(last, arrTmp.end());
    if (first != arrTmp.begin()) arrTmp.erase(arrTmp.begin(), first);
    std::reverse(arrTmp.begin(), arrTmp.end()); 
    ret.clear();
    ret.setArray();
    ret.push_backV(arrTmp);
    return ret;
}
static UniValue listsinceblock(const JSONRPCRequest& request)