message: nydus-image: improve help message for unpack subcommand
  Improve help message for unpack subcommand.
diff: @@ -481,26 +481,26 @@ fn prepare_cmd_args(bti_string: String) -> ArgMatches<'static> {
  )
  .subcommand(
  SubCommand::with_name("unpack")
  - .about("Unpack nydus image layer to a tar file")
  + .about("Unpack a RAFS filesystem to a tar file")
  .arg(
  Arg::with_name("bootstrap")
  .long("bootstrap")
  .short("B")
  - .help("path to bootstrap file")
  + .help("path to RAFS bootstrap file")
  .required(true)
  .takes_value(true))
  .arg(
  Arg::with_name("blob")
  .long("blob")
  .short("b")
  - .help("path to blob file")
  + .help("path to RAFS data blob file")
  .required(false)
  .takes_value(true)
  )
  .arg(
  Arg::with_name("output")
  .long("output")
  - .help("path to output tar file")
  + .help("path for output tar file")
  .required(true)
  .takes_value(true)
  )
  @@ -763,8 +763,15 @@ impl Command {
  fn unpack(args: &clap::ArgMatches) -> Result<()> {
  let bootstrap = args.value_of("bootstrap").expect("pass in bootstrap");
  - let blob = args.value_of("blob");
  + if bootstrap.is_empty() {
  + return Err(anyhow!("invalid empty --bootstrap option"));
  + }
  let output = args.value_of("output").expect("pass in output");
  + if output.is_empty() {
  + return Err(anyhow!("invalid empty --output option"));
  + }
  +
  + let blob = args.value_of("blob");
  let unpacker =
  OCIUnpacker::new(bootstrap, blob, output).with_context(|| "fail to create unpacker")?;
  