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

RPCResult::Type::OBJ, "", "", Cat(Cat<std::vector<RPCResult>>(
                    {
                        {RPCResult::Type::STR_AMOUNT, "amount", "The amount in " + CURRENCY_UNIT},
                        {RPCResult::Type::STR_AMOUNT, "fee", "The amount of the fee in " + CURRENCY_UNIT + ". This is negative and only available for the\n"
                                     "'send' category of transactions."},
                    },
                    TransactionDescriptionString()),
                    {
                        {RPCResult::Type::ARR, "details", "",
                        {
                            {RPCResult::Type::OBJ, "", "",
                            {
                                {RPCResult::Type::BOOL, "involvesWatchonly", "Only returns true if imported addresses were involved in transaction."},
                                {RPCResult::Type::STR, "address", "The bitcoin address involved in the transaction."},
                                {RPCResult::Type::STR, "category", "The transaction category.\n"
                                    "\"send\"                  Transactions sent.\n"
                                    "\"receive\"               Non-coinbase transactions received.\n"
                                    "\"generate\"              Coinbase transactions received with more than 100 confirmations.\n"
                                    "\"immature\"              Coinbase transactions received with 100 or fewer confirmations.\n"
                                    "\"orphan\"                Orphaned coinbase transactions received."},
                                {RPCResult::Type::STR_AMOUNT, "amount", "The amount in " + CURRENCY_UNIT},
                                {RPCResult::Type::STR, "label", "A comment for the address/transaction, if any"},
                                {RPCResult::Type::NUM, "vout", "the vout value"},
                                {RPCResult::Type::STR_AMOUNT, "fee", "The amount of the fee in " + CURRENCY_UNIT + ". This is negative and only available for the \n"
                                    "'send' category of transactions."},
                                {RPCResult::Type::BOOL, "abandoned", "'true' if the transaction has been abandoned (inputs are respendable). Only available for the \n"
                                     "'send' category of transactions."},
                            }},
                        }},
                        {RPCResult::Type::STR_HEX, "hex", "Raw data for transaction"},