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