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

{ "wallet",             &dumpprivkey,                    },
    { "wallet",             &dumpwallet,                     },
    { "wallet",             &encryptwallet,                  },
    { "wallet",             &getaddressesbylabel,            },
    { "wallet",             &getaddressinfo,                 },
    { "wallet",             &getbalance,                     },
    { "wallet",             &getnewaddress,                  },
    { "wallet",             &getrawchangeaddress,            },
    { "wallet",             &getreceivedbyaddress,           },
    { "wallet",             &getreceivedbylabel,             },
    { "wallet",             &gettransaction,                 },
    { "wallet",             &getunconfirmedbalance,          },
    { "wallet",             &getbalances,                    },
    { "wallet",             &getwalletinfo,                  },
    { "wallet",             &importaddress,                  },
    { "wallet",             &importdescriptors,              },
    { "wallet",             &importmulti,                    },
    { "wallet",             &importprivkey,                  },
    { "wallet",             &importprunedfunds,              },
    { "wallet",             &importpubkey,                   },
    { "wallet",             &importwallet,                   },
    { "wallet",             &keypoolrefill,                  },
    { "wallet",             &listaddressgroupings,           },
    { "wallet",             &listdescriptors,                },
    { "wallet",             &listlabels,                     },
    { "wallet",             &listlockunspent,                },
    { "wallet",             &listreceivedbyaddress,          },
    { "wallet",             &listreceivedbylabel,            },
    { "wallet",             &listsinceblock,                 },
    { "wallet",             &listtransactions,               },