`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();
    CWallet wallet(*chain, WalletLocation(), WalletDatabase::CreateDummy());
    CWalletTx wtx(&wallet, m_coinbase_txns.back());
    auto locked_chain = chain->lock();
    LOCK(wallet.cs_wallet);
    wtx.hashBlock = chainActive.Tip()->GetBlockHash();
    wtx.nIndex = 0;
    
    
    BOOST_CHECK_EQUAL(wtx.GetImmatureCredit(*locked_chain), 0);
    
    
    wtx.MarkDirty();
    wallet.AddKeyPubKey(coinbaseKey, coinbaseKey.GetPubKey());
    BOOST_CHECK_EQUAL(wtx.GetImmatureCredit(*locked_chain), 50*COIN);
}
static int64_t AddTx(CWallet& wallet, uint32_t lockTime, int64_t mockTime, int64_t blockTime)