`make cmd` does not fail when a good example returns errors

[jamescooke]

As part of the test run, the tox command cmd runs make cmd. This runs the command line interface python -m flake8_aaa against all the good example files. If there is an error in the good examples, then an error will be returned:

=== examples/good/test_comments_aaa05.py ===
------+------------------------------------------------------------------------
 4 DEF|def test_arrange():
       ^ AAA01 no Act block found in test
 5 ???|    x = 3
 6 ???|    # Let's make a 3, 4, 5 triangle
 7 ???|    y = 4
 8 BL |
 9 ???|    sresult = x ** 2 + y ** 2
10 BL |
11 ???|    assert result == 25
------+------------------------------------------------------------------------
    1 | ERROR

This should then generate an error, and break tox, however it doesn't and tox returns OK:

  py36-cmd: commands succeeded
  congratulations :)