Fix exception handling of `before_import` in `import_data`

Set `traceback.format_exc` limit to 2 in exception handling of `before_import`  in `import_data` method on `Resource`.

- Before it was traceback, which causing the following error: `unorderable types: int() < traceback()`
https://www.evernote.com/shard/s170/sh/734438dc-5db4-42ea-831f-55811ce87699/6cd97b6ffc5321a3092c6618202ca6df

Signed-off-by: Murat Akbal <murat@udemy.com>