`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.
{
    
    TestChain100Setup test;
    for (int i = 0; i < 5; ++i) {
        test.CreateAndProcessBlock({}, GetScriptForRawPubKey(test.coinbaseKey.GetPubKey()));
    }
    auto chain = interfaces::MakeChain();
    std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(*chain, WalletLocation(), WalletDatabase::CreateMock());
    bool firstRun;
    wallet->LoadWallet(firstRun);
    {
        LOCK(wallet->cs_wallet);
        wallet->SetAddressBook(GetDestinationForKey(test.coinbaseKey.GetPubKey(), wallet->m_default_address_type), "", "receive");
        wallet->AddKeyPubKey(test.coinbaseKey, test.coinbaseKey.GetPubKey());
    }
    {
        auto locked_chain = wallet->chain().lock();
        WalletRescanReserver reserver(wallet.get());