Raise custom error in command-line parsing

I'm using the <a href="http://golang.org/pkg/flag/">flag</a> module to parse my flags, but want to have at least one positional argument. How do I show the usage help when not enough positional arguments are present, as I would in python with parser.error?

Currently, I am manually calling os.Exit, but that feels really cumbersome for what should be a simple error:

package main
import "flag"
import "fmt"
import "os"
func main() {
  flag.Parse()
  if flag.NArg() != 1 {
  	println("This program needs exactly one argument")
  	flag.Usage()
  	os.Exit(2)
  }
  fmt.Printf("You entered %d characters", len(flag.Args()[0]))
}

To do things like this, I use the log package.

package main
import "flag"
import "fmt"
import "os"
import "log"
func main() {
  flag.Parse()
  if flag.NArg() != 1 {
    log.Fatalln("This program needs exactly one argument")
  }
  fmt.Printf("You entered %d characters", len(flag.Args()))
}

log.Fatal() and it's sister methods (log.Fatalln(), log.Fatalf() etc) are all helpers that simply do log.Print() and then follow it up with os.exit(1).

Edit -- Adding link
http://golang.org/pkg/log/#Fatalln