print usage message if not in interactive mode, and no rcon commands passed

cmd/q3rcon/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -16,6 +17,12 @@ import (
 
 var interactive bool
 
+func exit(err error) {
+	_, _ = fmt.Fprintf(os.Stderr, "Error: %s\n", err)
+	flag.Usage()
+	os.Exit(1)
+}
+
 func main() {
 	var (
 		host     string
@@ -57,6 +64,11 @@ func main() {
 		return
 	}
 
+	if len(flag.Args()) == 0 {
+		err = errors.New("no rcon commands passed")
+		exit(err)
+	}
+
 	for _, arg := range flag.Args() {
 		resp, err := rcon.Send(arg)
 		if err != nil {