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

{
    if (err_string.length() > 0) {
        return JSONRPCError(RPCErrorFromTransactionError(terr), err_string);
    } else {
        return JSONRPCError(RPCErrorFromTransactionError(terr), TransactionErrorString(terr).original);
    }
}
/**
 * A pair of strings that can be aligned (through padding) with other Sections
 * later on
 */
struct Section {
    Section(const std::string& left, const std::string& right)