Add broken config file to test error handling

ConfigFileFinder should absolutely handle broken/invalid config files
by refusing to try to parse them. Here we catch the ParsingError,
log the exception, and then return normally. The RawConfigParser
instance is perfectly valid still and will behave as if nothing had
been read and we just need to indicate that we didn't find any files
worthy of reading.

Related to: https://github.com/PyCQA/pycodestyle/issues/506

flake8/options/config.py

@@ -52,7 +52,13 @@ class ConfigFileFinder(object):
     @staticmethod
     def _read_config(files):
         config = configparser.RawConfigParser()
-        found_files = config.read(files)
+        try:
+            found_files = config.read(files)
+        except configparser.ParsingError:
+            LOG.exception("There was an error trying to parse a config "
+                          "file. The files we were attempting to parse "
+                          "were: %r", files)
+            found_files = []
         return (config, found_files)
 
     def cli_config(self, files):

tests/fixtures/config_files/broken.ini

@@ -0,0 +1,9 @@
+[flake8]
+exclude =
+<<<<<<< 642f88cb1b6027e184d9a662b255f7fea4d9eacc
+    tests/fixtures/,
+=======
+    tests/,
+>>>>>>> HEAD
+    docs/
+ignore = D203

tests/unit/test_config_file_finder.py

@@ -10,6 +10,7 @@ import mock
 import pytest
 
 CLI_SPECIFIED_FILEPATH = 'tests/fixtures/config_files/cli-specified.ini'
+BROKEN_CONFIG_PATH = 'tests/fixtures/config_files/broken.ini'
 
 
 def test_uses_default_args():
@@ -115,3 +116,12 @@ def test_local_configs():
     finder = config.ConfigFileFinder('flake8', None, [])
 
     assert isinstance(finder.local_configs(), configparser.RawConfigParser)
+
+
+@pytest.mark.parametrize('files', [
+    [BROKEN_CONFIG_PATH],
+    [CLI_SPECIFIED_FILEPATH, BROKEN_CONFIG_PATH],
+])
+def test_read_config_catches_broken_config_files(files):
+    """Verify that we do not allow the exception to bubble up."""
+    assert config.ConfigFileFinder._read_config(files)[1] == []