`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.
{
    CMutableTransaction tx;
    tx.nLockTime = lockTime;
    SetMockTime(mockTime);
    CBlockIndex* block = nullptr;
    if (blockTime > 0) {
        auto locked_chain = wallet.chain().lock();
        auto inserted = mapBlockIndex.emplace(GetRandHash(), new CBlockIndex);
        assert(inserted.second);
        const uint256& hash = inserted.first->first;
        block = inserted.first->second;
        block->nTime = blockTime;
        block->phashBlock = &hash;
    }
    CWalletTx wtx(&wallet, MakeTransactionRef(tx));
    if (block) {
        wtx.SetMerkleBranch(block, 0);
    }
    {
        LOCK(cs_main);
        wallet.AddToWallet(wtx);
    }
    LOCK(wallet.cs_wallet);
    return wallet.mapWallet.at(wtx.GetHash()).nTimeSmart;
}
BOOST_AUTO_TEST_CASE(ComputeTimeSmart)