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

const Type type,
        const std::string m_key_name,
        const std::string description,
        const std::vector<RPCResult> inner = {})
        : RPCResult{type, m_key_name, false, description, inner} {}
    
    void ToSections(Sections& sections, OuterType outer_type = OuterType::NONE, const int current_indent = 0) const;
    
    std::string ToStringObj() const;
    
    std::string ToDescriptionString() const;
    
    bool MatchesType(const UniValue& result) const;
};
struct RPCResults {
    const std::vector<RPCResult> m_results;
    RPCResults(RPCResult result)