`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 chain = interfaces::MakeChain();
    
    
    const int64_t BLOCK_TIME = chainActive.Tip()->GetBlockTimeMax() + 5;
    SetMockTime(BLOCK_TIME);
    m_coinbase_txns.emplace_back(CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey())).vtx[0]);
    m_coinbase_txns.emplace_back(CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey())).vtx[0]);
    
    
    const int64_t KEY_TIME = BLOCK_TIME + TIMESTAMP_WINDOW;
    SetMockTime(KEY_TIME);
    m_coinbase_txns.emplace_back(CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey())).vtx[0]);
    auto locked_chain = chain->lock();
    std::string backup_file = (SetDataDir("importwallet_rescan") / "wallet.backup").string();
    
    {
        std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(*chain, WalletLocation(), WalletDatabase::CreateDummy());
        LOCK(wallet->cs_wallet);
        wallet->mapKeyMetadata[coinbaseKey.GetPubKey().GetID()].nCreateTime = KEY_TIME;