message: improve error message for when subcommand contains `+toolchain`
diff: @@ -168,8 +168,9 @@ 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 = 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)
  + anyhow::format_err!("no such subcommand: `{}`{}\n\n\tCargo does not handle `+toolchain` directives itself.\n\tDid you mean to run `cargo` through `rustup` instead?\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)
  };
  