`help_msg` is needed even when `parse` raises errors.

[Milky2018]

Now we can only get the help_msg from parse like:

let help_msg = try {
  parser.parse(value, cli_args[:])
} catch {
  e => {
    println("Error: \{e}")
    return
  }
}

But we also need:

try {
  parser.parse(value, cli_args[:])
} catch {
  e => {
    println(help_msg)
    return
  }
}

Although there is public Parser::gen_help_message, it requires arguments which should not be provided by the users.