`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.
reserver.reserve();
        const CBlockIndex* stop_block;
        BOOST_CHECK_EQUAL(wallet.ScanForWalletTransactions(oldTip, nullptr, reserver, stop_block), CWallet::ScanResult::SUCCESS);
        BOOST_CHECK_EQUAL(stop_block, null_block);
        BOOST_CHECK_EQUAL(wallet.GetImmatureBalance(), 100 * COIN);
    }
    
    PruneOneBlockFile(oldTip->GetBlockPos().nFile);
    UnlinkPrunedFiles({oldTip->GetBlockPos().nFile});
    
    
    {
        CWallet wallet(*chain, WalletLocation(), WalletDatabase::CreateDummy());
        AddKey(wallet, coinbaseKey);
        WalletRescanReserver reserver(&wallet);
        reserver.reserve();
        const CBlockIndex* stop_block;
        BOOST_CHECK_EQUAL(wallet.ScanForWalletTransactions(oldTip, nullptr, reserver, stop_block), CWallet::ScanResult::FAILURE);
        BOOST_CHECK_EQUAL(oldTip, stop_block);
        BOOST_CHECK_EQUAL(wallet.GetImmatureBalance(), 50 * COIN);
    }
    
    
    
    {
        std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(*chain, WalletLocation(), WalletDatabase::CreateDummy());
        AddWallet(wallet);
        UniValue keys;
        keys.setArray();
        UniValue key;