Exit codes

With `error_code: normal` `stylish-haskell` doesn't return a non-zero exit code if there are formatting errors. It does so when `error_code: error_on_format`. With the former, errors in all files are reported; with the latter, only the first one. This is not very useful when scripting or in CI where one wants to see all errors (especially if the `--inplace` option is used) but also error if there are any formatting errors.

`error_code: normal` should report a non-zero exit code if there was at least one error. If that would be accepted, renaming the option might also be a good idea (e.g., `continue_on_error: Bool` or something similar).