Add body and index to BatchSendError.failures

* Add :message_body and :batch_index to entries in BatchSendError.failures.

* Updates specs to reflect this

Updates specs to reflect the changes.
Update documentation for BatchSendError.

Note that the previous documentation was copied from
BatchChangeVisibilityError and contained inaccuracies.
Merge pull request #338 from jjmason/batch_error_additions

Include information in SQS BatchSendError to facilitate retries