Ruler: Rule group not found API message

We copied over the ruler code as part of #5089, and it seems that at the same time we stopped depending on the object storage implementation of Cortex. Turns out, there are (slight) differences between the clients which meant that response returned by the API when we try to get a rule group that is not found had changed.

This ensures that is consistent with the error assertion that we have in the code for the ruler.

pkg/storage/chunk/gcp/gcs_object_client.go

@@ -135,6 +135,9 @@ func (s *GCSObjectClient) GetObject(ctx context.Context, objectKey string) (io.R
 func (s *GCSObjectClient) getObject(ctx context.Context, objectKey string) (rc io.ReadCloser, size int64, err error) {
 	reader, err := s.getsBuckets.Object(objectKey).NewReader(ctx)
 	if err != nil {
+		if err == storage.ErrObjectNotExist {
+			return nil, 0, chunk.ErrStorageObjectNotFound
+		}
 		return nil, 0, err
 	}