Typo in `DownloadMixin` error handling

https://github.com/transifex/transifex-python/blob/82ac9969c950f40b9c2fb86af1de3d31aaee9cf4/transifex/api/__init__.py#L26
should read:
```python
                raise DownloadException(download.errors[0]["detail"], download.errors)
```
