`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.
{
    AssertLockHeld(cs_main);
    AssertLockHeld(cs_wallet);
    std::map<CTxDestination, std::vector<COutput>> result;
    std::vector<COutput> availableCoins;
    AvailableCoins(locked_chain, availableCoins);
    for (const COutput& coin : availableCoins) {
        CTxDestination address;
        if (coin.fSpendable &&
            ExtractDestination(FindNonChangeParentOutput(*coin.tx->tx, coin.i).scriptPubKey, address)) {
            result[address].emplace_back(std::move(coin));
        }
    }
    std::vector<COutPoint> lockedCoins;
    ListLockedCoins(lockedCoins);
    for (const COutPoint& output : lockedCoins) {
        auto it = mapWallet.find(output.hash);
        if (it != mapWallet.end()) {