Exit non-zero if something goes wrong during a run

If we handle an exception, or early exit, or really anything, we should
exit non-zero (and we used to). This was a minor oversight.

Closes #209
Closes #248

tests/unit/test_application.py

@@ -0,0 +1,62 @@
+"""Tests for the Application class."""
+import optparse
+
+import mock
+import pytest
+
+from flake8.main import application as app
+
+
+def options(**kwargs):
+    """Generate optparse.Values for our Application."""
+    kwargs.setdefault('verbose', 0)
+    kwargs.setdefault('output_file', None)
+    kwargs.setdefault('count', False)
+    kwargs.setdefault('exit_zero', False)
+    return optparse.Values(kwargs)
+
+
+@pytest.mark.parametrize(
+    'result_count, catastrophic, exit_zero', [
+        (0, True, True),
+        (2, False, True),
+        (2, True, True),
+    ]
+)
+def test_exit_does_not_raise(result_count, catastrophic, exit_zero):
+    """Verify Application.exit doesn't raise SystemExit."""
+    with mock.patch('flake8.options.manager.OptionManager') as optionmanager:
+        optmgr = optionmanager.return_value = mock.Mock()
+        optmgr.parse_known_args.return_value = (options(), [])
+        application = app.Application()
+
+    application.result_count = result_count
+    application.catastrophic_failure = catastrophic
+    application.options = options(exit_zero=exit_zero)
+
+    assert application.exit() is None
+
+
+@pytest.mark.parametrize(
+    'result_count, catastrophic, exit_zero, value', [
+        (0, False, False, False),
+        (0, True, False, True),
+        (2, False, False, True),
+        (2, True, False, True),
+    ]
+)
+def test_exit_does_raise(result_count, catastrophic, exit_zero, value):
+    """Verify Application.exit doesn't raise SystemExit."""
+    with mock.patch('flake8.options.manager.OptionManager') as optionmanager:
+        optmgr = optionmanager.return_value = mock.Mock()
+        optmgr.parse_known_args.return_value = (options(), [])
+        application = app.Application()
+
+    application.result_count = result_count
+    application.catastrophic_failure = catastrophic
+    application.options = options(exit_zero=exit_zero)
+
+    with pytest.raises(SystemExit) as excinfo:
+        application.exit()
+
+    assert excinfo.value.args[0] is value