9db36f7ba3
Update the command matcher for the server command to log a more helpful
message when it's called without a subcommand.
Before:
```sh
$ target/debug/grin server
:: Some(ArgMatches { args: {}, subcommand: None, usage: Some("USAGE:\n grin server [OPTIONS] [SUBCOMMAND]") })
```
After:
```sh
$ target/debug/grin server
Subcommand required, use 'grin help server' for details
```