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 obj(UniValue::VOBJ);
        CPubKey pubkey;
        if (provider && provider->GetPubKey(ToKeyID(id), pubkey)) {
            obj.pushKV("pubkey", HexStr(pubkey));
        }
        return obj;
    }
    UniValue operator()(const WitnessV0ScriptHash& id) const
    {
        UniValue obj(UniValue::VOBJ);
        CScript subscript;
        CRIPEMD160 hasher;
        uint160 hash;
        hasher.Write(id.begin(), 32).Finalize(hash.begin());
        if (provider && provider->GetCScript(CScriptID(hash), subscript)) {
            ProcessSubScript(subscript, obj);
        }
        return obj;
    }
    UniValue operator()(const WitnessUnknown& id) const { return UniValue(UniValue::VOBJ); }