keys.js potential for uncaught exception

[rclark]

Sometimes it appears that the aws-sdk can return no error object, and also provide a data object where .Contents is undefined. In that case we would throw an unhandled error in the callback when we try and .slice the anticipated array.

s3.listObjects(params, function(err, data) {
  assert.notOk(err, 'there is no error');
  assert.notOk(data.Contents, 'there is also no information');
});

cc @yhahn

[rclark]

Upstream aws/aws-sdk-js#1216