values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    UniValue arr{UniValue::VARR};
    for (int i{0}; i < int(m_args.size()); ++i) {
        const auto& arg = m_args.at(i);
        std::vector<std::string> arg_names;
        boost::split(arg_names, arg.m_names, boost::is_any_of("|"));
        for (const auto& arg_name : arg_names) {
            UniValue map{UniValue::VARR};
            map.push_back(m_name);
            map.push_back(i);
            map.push_back(arg_name);
            map.push_back(arg.m_type == RPCArg::Type::STR ||
                          arg.m_type == RPCArg::Type::STR_HEX);
            arr.push_back(map);
        }
    }
    return arr;
}
std::string RPCArg::GetFirstName() const