fix: better help for invalid command args (#95)

Instead of handling it like an internal error, present the help message
as we do in _get_dispatcher() (for e.g. invalid commands) and return the
same return code.

Fixes #89

craft_application/application.py

@@ -280,7 +280,7 @@ def _get_dispatcher(self) -> _Dispatcher:
 
         return dispatcher
 
-    def run(self) -> int:
+    def run(self) -> int:  # noqa: PLR0912 (too many branches due to error handling)
         """Bootstrap and run the application."""
         dispatcher = self._get_dispatcher()
         craft_cli.emit.trace("Preparing application...")
@@ -315,6 +315,10 @@ def run(self) -> int:
                 # command runs in inner instance
                 self.services.project = self.project
                 return_code = dispatcher.run() or 0
+        except craft_cli.ArgumentParsingError as err:
+            print(err, file=sys.stderr)  # to stderr, as argparse normally does
+            craft_cli.emit.ended_ok()
+            return_code = 64  # Command line usage error from sysexits.h
         except KeyboardInterrupt as err:
             self._emit_error(craft_cli.CraftError("Interrupted."), cause=err)
             return_code = 128 + signal.SIGINT

tests/integration/test_application.py

@@ -15,6 +15,7 @@
 import argparse
 import pathlib
 import shutil
+import textwrap
 
 import craft_application
 import craft_cli
@@ -207,3 +208,25 @@ def test_get_command_help(monkeypatch, emitter, capsys, app, cmd, help_param):
     stdout, stderr = capsys.readouterr()
 
     assert f"testcraft {cmd} [options]" in stderr
+
+
+def test_invalid_command_argument(monkeypatch, capsys, app):
+    """Test help output when passing an invalid argument to a command."""
+    monkeypatch.setattr("sys.argv", ["testcraft", "pack", "--invalid-argument"])
+
+    return_code = app.run()
+
+    assert return_code == 64  # noqa: PLR2004 (Magic number used in comparison)
+
+    expected_stderr = textwrap.dedent(
+        """\
+        Usage: testcraft [options] command [args]...
+        Try 'testcraft pack -h' for help.
+
+        Error: unrecognized arguments: --invalid-argument
+
+    """
+    )
+
+    stdout, stderr = capsys.readouterr()
+    assert stderr == expected_stderr

tests/unit/test_application.py

@@ -370,6 +370,11 @@ def test_run_success_managed_inside_managed(
         (craft_parts.PartsError("unable to pull"), 1, "unable to pull\n"),
         (craft_providers.ProviderError("fail to launch"), 1, "fail to launch\n"),
         (Exception(), 70, "testcraft internal error: Exception()\n"),
+        (
+            craft_cli.ArgumentParsingError("Argument parsing error"),
+            64,
+            "Argument parsing error\n",
+        ),
     ],
 )
 def test_run_error(