`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.
{
    nIndex = -1;
    keypool.vchPubKey = CPubKey();
    {
        LOCK(cs_wallet);
        if (!IsLocked())
            TopUpKeyPool();
        bool fReturningInternal = IsHDEnabled() && CanSupportFeature(FEATURE_HD_SPLIT) && fRequestedInternal;
        bool use_split_keypool = set_pre_split_keypool.empty();
        std::set<int64_t>& setKeyPool = use_split_keypool ? (fReturningInternal ? setInternalKeyPool : setExternalKeyPool) : set_pre_split_keypool;
        
        if (setKeyPool.empty()) {
            return false;
        }
        WalletBatch batch(*database);
        auto it = setKeyPool.begin();
        nIndex = *it;
        setKeyPool.erase(it);
        if (!batch.ReadPool(nIndex, keypool)) {
            throw std::runtime_error(std::string(__func__) + ": read failed");
        }
        if (!HaveKey(keypool.vchPubKey.GetID())) {
            throw std::runtime_error(std::string(__func__) + ": unknown key in key pool");
        }
        
        if (use_split_keypool && keypool.fInternal != fReturningInternal) {
            throw std::runtime_error(std::string(__func__) + ": keypool entry misclassified");
        }
        if (!keypool.vchPubKey.IsValid()) {
            throw std::runtime_error(std::string(__func__) + ": keypool entry invalid");