Raise error if CRL download fails.

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

atst/domain/csp/files.py

@@ -9,6 +9,10 @@ from libcloud.storage.providers import get_driver
 from atst.domain.exceptions import UploadError
 
 
+class CSPFileError(Exception):
+    pass
+
+
 class FileProviderInterface:
     _PERMITTED_MIMETYPES = ["application/pdf", "image/png"]
 
@@ -99,14 +103,23 @@ class RackspaceCRLProvider(CRLProviderInterface):
         )
         self._crl_dir = app.config.get("CRL_STORAGE_CONTAINER")
         self._object_name = app.config.get("STORAGE_CRL_ARCHIVE_NAME")
+        self._object = None
+
+    @property
+    def object(self):
+        if self._object is None:
+            self._object = self.container.get_object(object_name=self._object_name)
+
+        return self._object
 
     def sync_crls(self):
         if not os.path.exists(self._crl_dir):
             os.mkdir(self._crl_dir)
 
-        obj = self.container.get_object(object_name=self._object_name)
         with TemporaryDirectory() as tempdir:
             dl_path = os.path.join(tempdir, self._object_name)
-            obj.download(dl_path, overwrite_existing=True)
+            success = self.object.download(dl_path, overwrite_existing=True)
+            if not success:
+                raise CSPFileError("The CRL package was not downloaded")
             archive = tarfile.open(dl_path, "r:bz2")
             archive.extractall(self._crl_dir)