`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.
#ifndef BITCOIN_WALLET_WALLET_H
#define BITCOIN_WALLET_WALLET_H
#include <amount.h>
#include <interfaces/chain.h>
#include <outputtype.h>
#include <policy/feerate.h>
#include <streams.h>
#include <tinyformat.h>
#include <ui_interface.h>
#include <util/strencodings.h>
#include <validationinterface.h>
#include <script/ismine.h>
#include <script/sign.h>
#include <util/system.h>
#include <wallet/crypter.h>
#include <wallet/coinselection.h>
#include <wallet/walletdb.h>
#include <wallet/walletutil.h>
#include <algorithm>
#include <atomic>
#include <map>
#include <memory>
#include <set>