More small tidyups

- switch to using sys.exit instead of exit
 - always exit with error-code 1
 - don't bother testing error-codes
 - documentation wording tweak

tests/cli/test_pinout.py

@@ -15,8 +15,6 @@ from gpiozero.cli import pinout
 def test_args_incorrect():
     with pytest.raises(SystemExit) as ex:
         pinout.parse_args(['--nonexistentarg'])
-    assert ex.value.code == 2
-
 
 def test_args_color():
     args = pinout.parse_args([])
@@ -26,15 +24,12 @@ def test_args_color():
     args = pinout.parse_args(['--monochrome'])
     assert args.color is False
 
-
 def test_args_revision():
     args = pinout.parse_args(['--revision', '000d'])
     assert args.revision == '000d'
 
-
 def test_help(capsys):
     with pytest.raises(SystemExit) as ex:
         pinout.parse_args(['--help'])
     out, err = capsys.readouterr()
     assert 'GPIO pinout' in out
-    assert ex.value.code == 0