Pass image_id to ImageDownloadFailed

When an ImageDownloadFailed exception is created in the
GlanceImageService, it's being passed the context whereas what
makes sense for the error message is the image_id.  This patch
makes the change and adds a unit test for a previous fix for
Bug #1798147 (which was the real point of this patch).

Change-Id: I6d8dedfd056add3414f8f4bf7f7279eae4763286
Closes-bug: #1811184

cinder/image/glance.py

@@ -359,7 +359,7 @@ class GlanceImageService(object):
 
         if image_chunks is None:
             raise exception.ImageDownloadFailed(
-                image_href=context, reason=_('image contains no data.'))
+                image_href=image_id, reason=_('image contains no data.'))
 
         if not data:
             return image_chunks

cinder/tests/unit/image/test_glance.py

@@ -16,6 +16,7 @@
 
 import datetime
 import itertools
+import six
 
 import ddt
 import glanceclient.exc
@@ -663,6 +664,20 @@ class TestGlanceImageService(test.TestCase):
         self.flags(glance_num_retries=1)
         service.download(self.context, image_id, writer)
 
+    def test_download_no_data(self):
+        class MyGlanceStubClient(glance_stubs.StubGlanceClient):
+            """Returns None instead of an iterator."""
+            def data(self, image_id):
+                return None
+
+        client = MyGlanceStubClient()
+        service = self._create_image_service(client)
+        image_id = 'fake-image-uuid'
+        e = self.assertRaises(exception.ImageDownloadFailed, service.download,
+                              self.context, image_id)
+        self.assertIn('image contains no data', six.text_type(e))
+        self.assertIn(image_id, six.text_type(e))
+
     def test_client_forbidden_converts_to_imagenotauthed(self):
         class MyGlanceStubClient(glance_stubs.StubGlanceClient):
             """A client that raises a Forbidden exception."""