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 std::array<unsigned char, 32> privkey_bytes = {
         j
         d
        0xD9, 0x7F, 0x51, 0x08, 0xF1, 0x1C, 0xDA, 0x6E,
        0xEE, 0xBA, 0xAA, 0x42, 0x0F, 0xEF, 0x07, 0x26,
        0xB1, 0xF8, 0x98, 0x06, 0x0B, 0x98, 0x48, 0x9F,
        0xA3, 0x09, 0x84, 0x63, 0xC0, 0x03, 0x28, 0x66
    };
    const std::string message = "Trust no one";
    const std::string expected_signature =
        "IPojfrX2dfPnH26UegfbGQQLrdK844DlHq5157/P6h57WyuS/Qsl+h/WSVGDF4MUi4rWSswW38oimDYfNNUBUOk=";
    CKey privkey;
    std::string generated_signature;
    BOOST_REQUIRE_MESSAGE(!privkey.IsValid(),
        "Confirm the private key is invalid");
    BOOST_CHECK_MESSAGE(!MessageSign(privkey, message, generated_signature),
        "Sign with an invalid private key");
    privkey.Set(privkey_bytes.begin(), privkey_bytes.end(), true);
    BOOST_REQUIRE_MESSAGE(privkey.IsValid(),
        "Confirm the private key is valid");
    BOOST_CHECK_MESSAGE(MessageSign(privkey, message, generated_signature),
        "Sign with a valid private key");
    BOOST_CHECK_EQUAL(expected_signature, generated_signature);
}
BOOST_AUTO_TEST_CASE(message_verify)