Output a more clear configuration error (#421)

* Output a more clear configuration error

* Performance related improvements in config read handling

* Check python 3.6 with travis

* Revert the .travis.yml py36 inclusion

* Added tests for input error handling

* Thanks isort, isorted it out

* Make exit() function a bit more aesthetic

* Single quote -> double quote docstrings

* Whitespace removed

test-plugin/test_error_handling.py

@@ -0,0 +1,73 @@
+import tempfile
+import typing
+
+import pytest
+
+from mypy_django_plugin.main import extract_django_settings_module
+
+TEMPLATE = """usage: (config)
+...
+[mypy.plugins.django_stubs]
+    django_settings_module: str (required)
+...
+(django-stubs) mypy: error: 'django_settings_module' is not set: {}
+"""
+
+
+@pytest.mark.parametrize(
+    'config_file_contents,message_part',
+    [
+        pytest.param(
+            None,
+            'mypy config file is not specified or found',
+            id='missing-file',
+        ),
+        pytest.param(
+            ['[not-really-django-stubs]'],
+            'no section [mypy.plugins.django-stubs]',
+            id='missing-section',
+        ),
+        pytest.param(
+            ['[mypy.plugins.django-stubs]',
+             '\tnot_django_not_settings_module = badbadmodule'],
+            'the setting is not provided',
+            id='missing-settings-module',
+        ),
+        pytest.param(
+            ['[mypy.plugins.django-stubs]'],
+            'the setting is not provided',
+            id='no-settings-given',
+        ),
+    ],
+)
+def test_misconfiguration_handling(capsys, config_file_contents, message_part):
+    #  type: (typing.Any, typing.List[str], str) -> None
+    """Invalid configuration raises `SystemExit` with a precise error message."""
+    with tempfile.NamedTemporaryFile(mode='w+') as config_file:
+        if not config_file_contents:
+            config_file.close()
+        else:
+            config_file.write('\n'.join(config_file_contents).expandtabs(4))
+            config_file.seek(0)
+
+        with pytest.raises(SystemExit, match='2'):
+            extract_django_settings_module(config_file.name)
+
+    error_message = TEMPLATE.format(message_part)
+    assert error_message == capsys.readouterr().err
+
+
+def test_correct_configuration() -> None:
+    """Django settings module gets extracted given valid configuration."""
+    config_file_contents = [
+        '[mypy.plugins.django-stubs]',
+        '\tsome_other_setting = setting',
+        '\tdjango_settings_module = my.module',
+    ]
+    with tempfile.NamedTemporaryFile(mode='w+') as config_file:
+        config_file.write('\n'.join(config_file_contents).expandtabs(4))
+        config_file.seek(0)
+
+        extracted = extract_django_settings_module(config_file.name)
+
+    assert extracted == 'my.module'