In the log I see things like:

    2017-09-13 03:54:33 init message: Rescanning...
    2017-09-13 03:54:33 Rescanning last 68616 blocks (from block 416339)...
    2017-09-13 03:54:40 AddToWallet xxxxx  update
    2017-09-13 03:55:33 Still rescanning. At block 418849. Progress=0.549138

I would be interested to see the wallet file name on each of these log lines.

Would there be any interest in merging such a PR?
std::string strAddress = request.params[0].get_str();
    CTxDestination dest = DecodeDestination(strAddress);
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    }
    auto keyid = GetKeyForDestination(*pwallet, dest);
    if (keyid.IsNull()) {
        throw JSONRPCError(RPC_TYPE_ERROR, "Address does not refer to a key");
    }
    CKey vchSecret;
    if (!pwallet->GetKey(keyid, vchSecret)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Private key for address " + strAddress + " is not known");
    }
    return EncodeSecret(vchSecret);
}
UniValue dumpwallet(const JSONRPCRequest& request)
{
    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() != 1)
        throw std::runtime_error(
            "dumpwallet \"filename\"\n"
            "\nDumps all wallet keys in a human-readable format to a server-side file. This does not allow overwriting existing files.\n"
            "Imported scripts are included in the dumpfile, but corresponding BIP173 addresses, etc. may not be added automatically by importwallet.\n"
            "Note that if your wallet contains keys which are not derived from your HD seed (e.g. imported keys), these are not covered by\n"
            "only backing up the seed itself, and must be backed up too (e.g. ensure you back up the whole dumpfile).\n"
            "\nArguments:\n"