Batch operation exception handling improvement and code clean up.

When an exception occurred for a batch operation, the response status
code for the message is set to the proper status code defined in
kinetic.proto (such as VERSION_MISMATCH). 

The subsequential response messages' status code
(PUT_RESPONSE/DELETE_RESPONSE) in the failed batch are set to
NOT_ATTEMPTED, and the END_BATCH_RESPONSE status is set to
INVALID_BATCH.