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

OBJ,
        ARR,
        STR,
        NUM,
        BOOL,
        NONE,
        ANY,        
        STR_AMOUNT, 
        STR_HEX,    
        OBJ_DYN,    
        ARR_FIXED,  
        NUM_TIME,   
        ELISION,    
    };
    const Type m_type;
    const std::string m_key_name;         
    const std::vector<RPCResult> m_inner; 
    const bool m_optional;
    const std::string m_description;
    const std::string m_cond;
    RPCResult(