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

m_fallback{std::move(fallback)},
          m_description{std::move(description)},
          m_oneline_description{std::move(oneline_description)},
          m_type_str{std::move(type_str)}
    {
        CHECK_NONFATAL(type == Type::ARR || type == Type::OBJ || type == Type::OBJ_USER_KEYS);
    }
    bool IsOptional() const;
    
    std::string GetFirstName() const;
    
    std::string GetName() const;
    /**
     * Return the type string of the argument.
     * Set oneline to allow it to be overridden by a custom oneline type string (m_oneline_description).
     */
    std::string ToString(bool oneline) const;
    /**
     * Return the type string of the argument when it is in an object (dict).
     * Set oneline to get the oneline representation (less whitespace)
     */
    std::string ToStringObj(bool oneline) const;
    /**
     * Return the description string, including the argument type and whether
     * the argument is required.
     */
    std::string ToDescriptionString() const;
};
struct RPCResult {
    enum class Type {