Improve error handling for missing arguments

[evan-goode]

1. Print "It could be a command provided by a plugin..." hint to stderr, not stdout
2. When dnf5 missingcommand --help is run, don't print the top-level help; instead print the "Unknown argument" error and exit with a non-zero code.

@jrohel let me know whether this looks good, if so I'll prepare some CI tests.

[jrohel]

$ dnf5 abc
Unknown argument "abc" for command "dnf5". Run "dnf5 --help" for a list of commands.
It could be a command provided by a plugin, try: dnf install dnf5-command(abc)

OK

$ dnf5/dnf5 --abc
Unknown argument "--abc" for command "dnf5". Add "--help" for more information about the arguments.

OK

$ dnf5/dnf5 --abc --help
Unknown argument "--abc" for command "dnf5". Add "--help" for more information about the arguments.

Fail. I added --help as advised. And I got again the same error message.

$ dnf5/dnf5 repo --abc
Unknown argument "--abc" for command "repo". Add "--help" for more information about the arguments.

OK

$ dnf5/dnf5 repo --abc --help
Usage:
  dnf5 [GLOBAL OPTIONS] repo [OPTIONS] <COMMAND> ...
                                
Query Commands:                 
  list                          List repositories
  info                          Print details about repositories
                                
Options:                        
  --forcearch=FORCEARCH         Force the use of a different architecture.

While dnf5/dnf5 --abc --help prints an error message without help, the subcommand prints help without an error message. What to unify it? For example, when there is an error and --help at the same time, print help and error message.

An example of code that prints what I have in mind:

            } catch (libdnf5::cli::ArgumentParserError & ex) {
                // Error during parsing arguments. Try to find "--help"/"-h".
                bool help_printed{false};
                for (int idx = 1; idx < argc; ++idx) {
                    if (strcmp(argv[idx], "-h") == 0 || strcmp(argv[idx], "--help") == 0) {
                        arg_parser.get_selected_command()->help();
                        help_printed = true;
                        break;
                    }
                }
                std::cerr << ex.what()
                          << (help_printed ? "." : _(". Add \"--help\" for more information about the arguments."))
                          << std::endl;
                // If the error is an unknown top-level command, suggest
                // installing a package that provides the command
                if (auto * unknown_arg_ex = dynamic_cast<libdnf5::cli::ArgumentParserUnknownArgumentError *>(&ex)) {
                    if (unknown_arg_ex->get_command() == "dnf5" && unknown_arg_ex->get_argument()[0] != '-') {
                        std::cerr
                            << fmt::format(
                                   "It could be a command provided by a plugin, try: dnf install dnf5-command({})",
                                   unknown_arg_ex->get_argument())
                            << std::endl;
                    }
                }
                return static_cast<int>(libdnf5::cli::ExitCode::ARGPARSER_ERROR);
            }

[jrohel]

I already described in previous the comment.

[evan-goode]

Sure, I like that. I think the team was concerned that the error would be obscured by printing the help, but IMO it's alright if the error is printed last.

[jrohel]

LGTM. Thank you.