fix(cli): show error/hide help for unsupported subcommands (#10760) (#12624)

Fixes #10760 

The coder CLI quietly accepts any subcommand arguments and silently swallows them.

Currently:
```sh
❯ coder | head -n5
coder v2.3.3+e491217

USAGE:
  coder [global-flags] <subcommand>
```

```sh
❯ coder idontexist | head -n5
coder v2.3.3+e491217

USAGE:
  coder [global-flags] <subcommand>
```

Now help output will not be show when there is an unknown subcommand error. Instead users will be given the command for the help output.

```sh
❯ coder idontexist
Encountered an error running "coder", see "coder --help" for more information 
error: unrecognized subcommand "idontexist"
```
```sh
❯ coder iexistbut idontexist
Encountered an error running "coder iexistbut", see "coder iexistbut --help" for more information 
error: unrecognized subcommand "idontexist"
```

Also this stuff: `Encountered an error running "coder iexistbut"... ` gets written to `os.Stdout` in `prettyErrorFormatter{w: os.Stderr, verbose: r.verbose}`, not sure how to test that output.

cli/portforward.go

@@ -69,10 +69,6 @@ func (r *RootCmd) portForward() *serpent.Command {
 				return xerrors.Errorf("parse port-forward specs: %w", err)
 			}
 			if len(specs) == 0 {
-				err = inv.Command.HelpHandler(inv)
-				if err != nil {
-					return xerrors.Errorf("generate help output: %w", err)
-				}
 				return xerrors.New("no port-forwards requested")
 			}