Don't store http responses from paginated results

This is really bad if you are paginating over a large
set of data (like s3 objects).

This was previously used to check for error responses, but this
is no longer used by anything.  The error handling is all event
based now (via `after-call` events).

Fixes https://github.com/aws/aws-cli/issues/326.  Note that one other
commit to the CLI is needed to cap memory usage and fix
https://github.com/aws/aws-cli/issues/326.