`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.
"\nResult:\n"
            "[ blockhashes ]     (array) hashes of blocks generated\n"
            "\nExamples:\n"
            "\nGenerate 11 blocks\n"
            + HelpExampleCli("generate", "11")
        );
    }
    if (!IsDeprecatedRPCEnabled("generate")) {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "The wallet generate rpc method is deprecated and will be fully removed in v0.19. "
            "To use generate in v0.18, restart bitcoind with -deprecatedrpc=generate.\n"
            "Clients should transition to using the node rpc method generatetoaddress\n");
    }
    int num_generate = request.params[0].get_int();
    uint64_t max_tries = 1000000;
    if (!request.params[1].isNull()) {
        max_tries = request.params[1].get_int();
    }
    std::shared_ptr<CReserveScript> coinbase_script;
    pwallet->GetScriptForMining(coinbase_script);
    
    if (!coinbase_script) {
        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
    }
    
    if (coinbase_script->reserveScript.empty()) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "No coinbase script available");
    }
    return generateBlocks(coinbase_script, num_generate, max_tries, true);
}
UniValue rescanblockchain(const JSONRPCRequest& request)