`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.
{
    LockAnnotation lock(::cs_main); 
    auto locked_chain = chain().lock();
    LOCK(cs_wallet);
    CAmount balance = 0;
    for (const auto& entry : mapWallet) {
        const CWalletTx& wtx = entry.second;
        const int depth = wtx.GetDepthInMainChain(*locked_chain);
        if (depth < 0 || !CheckFinalTx(*wtx.tx) || wtx.IsImmatureCoinBase(*locked_chain)) {
            continue;
        }
        
        
        CAmount debit = wtx.GetDebit(filter);
        const bool outgoing = debit > 0;
        for (const CTxOut& out : wtx.tx->vout) {
            if (outgoing && IsChange(out)) {
                debit -= out.nValue;
            } else if (IsMine(out) & filter && depth >= minDepth) {
                balance += out.nValue;
            }
        }
        
        if (outgoing) {
            balance -= debit;
        }
    }
    return balance;
}
CAmount CWallet::GetAvailableBalance(const CCoinControl* coinControl) const