`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.
{
private:
  
    static const uint256 ABANDON_HASH;
public:
    CTransactionRef tx;
    uint256 hashBlock;
    /* An nIndex == -1 means that hashBlock (in nonzero) refers to the earliest
     * block in the chain we know this or any in-wallet dependency conflicts
     * with. Older clients interpret nIndex == -1 as unconfirmed for backward
     * compatibility.
     */
    int nIndex;
    CMerkleTx()
    {
        SetTx(MakeTransactionRef());
        Init();
    }
    explicit CMerkleTx(CTransactionRef arg)
    {
        SetTx(std::move(arg));
        Init();
    }
    void Init()
    {
        hashBlock = uint256();
        nIndex = -1;
    }
    void SetTx(CTransactionRef arg)