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

{
    BOOST_CHECK_EQUAL(
        MessageVerify(
            "invalid address",
            "signature should be irrelevant",
            "message too"),
        MessageVerificationResult::ERR_INVALID_ADDRESS);
    BOOST_CHECK_EQUAL(
        MessageVerify(
            "3B5fQsEXEaV8v6U3ejYc8XaKXAkyQj2MjV",
            "signature should be irrelevant",
            "message too"),
        MessageVerificationResult::ERR_ADDRESS_NO_KEY);
    BOOST_CHECK_EQUAL(
        MessageVerify(