Fix a traceback when an invalid command-line flag or action is used (#768).

tests/end-to-end/test_invalid_flag.py

@@ -0,0 +1,11 @@
+import subprocess
+import sys
+
+
+def test_borgmatic_command_with_invalid_flag_shows_error_but_not_traceback():
+    output = subprocess.run(
+        'borgmatic -v 2 --invalid'.split(' '), stdout=subprocess.PIPE, stderr=subprocess.STDOUT
+    ).stdout.decode(sys.stdout.encoding)
+
+    assert 'Unrecognized argument' in output
+    assert 'Traceback' not in output