`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.
* scan was successful. FAILURE if a complete rescan was not possible (due to
 * pruning or corruption). USER_ABORT if the rescan was aborted before it
 * could complete.
 *
 * @pre Caller needs to make sure pindexStop (and the optional pindexStart) are on
 * the main chain after to the addition of any new keys you want to detect
 * transactions for.
 */
CWallet::ScanResult CWallet::ScanForWalletTransactions(const CBlockIndex* const pindexStart, const CBlockIndex* const pindexStop, const WalletRescanReserver& reserver, const CBlockIndex*& failed_block, bool fUpdate)
{
    int64_t nNow = GetTime();
    const CChainParams& chainParams = Params();
    assert(reserver.isReserved());
    if (pindexStop) {
        assert(pindexStop->nHeight >= pindexStart->nHeight);
    }
    const CBlockIndex* pindex = pindexStart;
    failed_block = nullptr;
    if (pindex) WalletLogPrintf("Rescan started from block %d...\n", pindex->nHeight);
    {
        fAbortRescan = false;
        ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), 0); 
        CBlockIndex* tip = nullptr;
        double progress_begin;
        double progress_end;
        {
            auto locked_chain = chain().lock();
            progress_begin = GuessVerificationProgress(chainParams.TxData(), pindex);
            if (pindexStop == nullptr) {
                tip = chainActive.Tip();