feat: improve the readability of oli command line error output

[nayihz]

This PR just polish the output of oli command line when no subcommand was input.
Before:

# ./oli
Error: not handled

After:

# ./oli
OpenDAL Command Line Interface

Usage: oli [OPTIONS] <COMMAND>

Commands:
  cat   display object content
  cp    copy
  ls    ls
  rm    remove object
  stat  show object metadata
  help  Print this message or the help of the given subcommand(s)

Options:
      --config <config>  Path to the config file [default: /root/.config/oli/config.toml]
  -h, --help             Print help
  -V, --version          Print version

[Xuanwo]

Thanks!