cloud/datastore: don't export an HTTP error type

We should either export such a type from the cloud package
and cast every raw response error to cloud.ErrHTTP or stop
exporting a type for such errors.

All recoverable errors should have custom error values
exported from the related package such as
datastore.ErrConcurrentTransaction. Therefore, I'm preferring
to lowercase this type.

Change-Id: Ifdd052d2bbd90b3e4c17f7d2afff025f68943a84