`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.
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() != 0) {
        throw std::runtime_error(
            "walletlock\n"
            "\nRemoves the wallet encryption key from memory, locking the wallet.\n"
            "After calling this method, you will need to call walletpassphrase again\n"
            "before being able to call any methods which require the wallet to be unlocked.\n"
            "\nExamples:\n"
            "\nSet the passphrase for 2 minutes to perform a transaction\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\" 120") +
            "\nPerform a send (requires passphrase set)\n"
            + HelpExampleCli("sendtoaddress", "\"1M72Sfpbz1BPpXFHz9m3CdqATR44Jvaydd\" 1.0") +
            "\nClear the passphrase since we are done before 2 minutes is up\n"
            + HelpExampleCli("walletlock", "") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("walletlock", "")
        );