Bug fix and test for throwing unhandled DynamoDbExceptions.

When transferring a batch, certain DynamoDbExceptions are handled (retry large request, retry items rejected due to throttling).  Any other DynamoDbException were silently ignored.  The bug was introduced in https://github.com/aws/aws-sdk-php/commit/c882b65ae9f713d3f67a0c34a141f19ba4fa90d7#diff-bbde4badee36d6ebc40b1c7820e9196a.

This allows those exceptions to be thrown so the user can handle it rather than wondering why some of their data didn't get transferred.