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

break;
            case feebumper::Result::INVALID_PARAMETER:
                throw JSONRPCError(RPC_INVALID_PARAMETER, errors[0].original);
                break;
            case feebumper::Result::WALLET_ERROR:
                throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
                break;
            default:
                throw JSONRPCError(RPC_MISC_ERROR, errors[0].original);
                break;
        }
    }
    UniValue result(UniValue::VOBJ);
    
    
    if (!want_psbt) {
        if (!feebumper::SignTransaction(*pwallet, mtx)) {
            throw JSONRPCError(RPC_WALLET_ERROR, "Can't sign transaction.");
        }
        uint256 txid;
        if (feebumper::CommitTransaction(*pwallet, hash, std::move(mtx), errors, txid) != feebumper::Result::OK) {
            throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
        }
        result.pushKV("txid", txid.GetHex());
    } else {
        PartiallySignedTransaction psbtx(mtx);
        bool complete = false;
        const TransactionError err = pwallet->FillPSBT(psbtx, complete, SIGHASH_ALL, false , true );
        CHECK_NONFATAL(err == TransactionError::OK);
        CHECK_NONFATAL(!complete);