`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.
{
public:
    ListCoinsTestingSetup()
    {
        CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
        wallet = MakeUnique<CWallet>(*m_chain, WalletLocation(), WalletDatabase::CreateMock());
        bool firstRun;
        wallet->LoadWallet(firstRun);
        AddKey(*wallet, coinbaseKey);
        WalletRescanReserver reserver(wallet.get());
        reserver.reserve();
        const CBlockIndex* const null_block = nullptr;
        const CBlockIndex* stop_block;
        BOOST_CHECK_EQUAL(wallet->ScanForWalletTransactions(chainActive.Genesis(), nullptr, reserver, stop_block), CWallet::ScanResult::SUCCESS);
        BOOST_CHECK_EQUAL(stop_block, null_block);
    }
    ~ListCoinsTestingSetup()
    {
        wallet.reset();
    }
    CWalletTx& AddTx(CRecipient recipient)