swapd should immediately exit with good error message when a subcommand is passed
It is easy to type `./bin/swapd` when you wanted to type `./bin/swapcli`.  You get something like this:
```
./bin/swapd get-offers
2023-01-25T03:41:18.100Z	ERROR	cmd	swapd/main.go:291	RPC/Websocket server exited: dial tcp 127.0.0.1:8545: connect: connection refused
```
If we have a non flag parameter on the `swapd` command line, we should just exit immediately with a clear error. We want the same behavior that the user already gets when they pass an unknown flag name.