`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.
{
    LOCK(cs_wallet);
    WalletBatch batch(*database, "r+", fFlushOnClose);
    uint256 hash = wtxIn.GetHash();
    
    std::pair<std::map<uint256, CWalletTx>::iterator, bool> ret = mapWallet.insert(std::make_pair(hash, wtxIn));
    CWalletTx& wtx = (*ret.first).second;
    wtx.BindWallet(this);
    bool fInsertedNew = ret.second;
    if (fInsertedNew) {
        wtx.nTimeReceived = GetAdjustedTime();
        wtx.nOrderPos = IncOrderPosNext(&batch);