message: handling if a subcommand looks like `+toolchain`. very early implementation; very basic and not final
diff: @@ -168,7 +168,12 @@ fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&str]) -> Cli
  None => {
  let suggestions = list_commands(config);
  let did_you_mean = closest_msg(cmd, suggestions.keys(), |c| c);
  - let err = anyhow::format_err!("no such subcommand: `{}`{}", cmd, did_you_mean);
  + let err = if cmd.contains('+') {
  + anyhow::format_err!("no such subcommand: `{}`{}\n\n\tCargo does not handle `+toolchain` directives.\n\tsuggestion: view a list of all installed Cargo subcommands using `cargo --list`", cmd, did_you_mean)
  + } else {
  + anyhow::format_err!("no such subcommand: `{}`{}\n\n\tsuggestion: view a list of all installed Cargo subcommands using `cargo --list`", cmd, did_you_mean)
  + };
  +
  return Err(CliError::new(err, 101));
  }
  };
  