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",             &listunspent,                    },
    { "wallet",             &listwalletdir,                  },
    { "wallet",             &listwallets,                    },
    { "wallet",             &loadwallet,                     },
    { "wallet",             &lockunspent,                    },
    { "wallet",             &removeprunedfunds,              },
    { "wallet",             &rescanblockchain,               },
    { "wallet",             &send,                           },
    { "wallet",             &sendmany,                       },
    { "wallet",             &sendtoaddress,                  },
    { "wallet",             &sethdseed,                      },
    { "wallet",             &setlabel,                       },
    { "wallet",             &settxfee,                       },
    { "wallet",             &setwalletflag,                  },
    { "wallet",             &signmessage,                    },
    { "wallet",             &signrawtransactionwithwallet,   },
    { "wallet",             &unloadwallet,                   },
    { "wallet",             &upgradewallet,                  },
    { "wallet",             &walletcreatefundedpsbt,         },
#ifdef ENABLE_EXTERNAL_SIGNER
    { "wallet",             &walletdisplayaddress,           },
#endif 
    { "wallet",             &walletlock,                     },
    { "wallet",             &walletpassphrase,               },
    { "wallet",             &walletpassphrasechange,         },
    { "wallet",             &walletprocesspsbt,              },
};
    return MakeSpan(commands);
}