The download method for Libcloud objects returns a boolean, which means
that our CRL download could fail silently. The RackspaceCRLProvider
would not raise an error until it tried to open the full path for the
downloaded resource. This checks the return status of the download call
and raises an error if the download failed. For reference:
https://libcloud.readthedocs.io/en/latest/storage/api.html#libcloud.storage.base.StorageDriver.download_object
