`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.
UniValue operator()(const CNoDestination& dest) const { return UniValue(UniValue::VOBJ); }
    UniValue operator()(const CKeyID& keyID) const
    {
        UniValue obj(UniValue::VOBJ);
        CPubKey vchPubKey;
        if (pwallet && pwallet->GetPubKey(keyID, vchPubKey)) {
            obj.pushKV("pubkey", HexStr(vchPubKey));
            obj.pushKV("iscompressed", vchPubKey.IsCompressed());
        }
        return obj;
    }
    UniValue operator()(const CScriptID& scriptID) const
    {
        UniValue obj(UniValue::VOBJ);
        CScript subscript;
        if (pwallet && pwallet->GetCScript(scriptID, subscript)) {
            ProcessSubScript(subscript, obj, IsDeprecatedRPCEnabled("validateaddress"));
        }
        return obj;
    }
    UniValue operator()(const WitnessV0KeyHash& id) const
    {
        UniValue obj(UniValue::VOBJ);
        CPubKey pubkey;
        if (pwallet && pwallet->GetPubKey(CKeyID(id), pubkey)) {
            obj.pushKV("pubkey", HexStr(pubkey));
        }
        return obj;
    }
    UniValue operator()(const WitnessV0ScriptHash& id) const