Add Error log to upload_blob func.

In order to have more info when upload_blob
fails, we add this log.
Learning from https://bugs.launchpad.net/glare/+bug/1742729
case, where this kind of info was missing.

Change-Id: If91e014692cbe6ad39c473eecef520b08881d53b

glare/engine.py

@@ -600,6 +600,7 @@ class Engine(object):
         except Exception:
             # if upload failed remove blob from db and storage
             with excutils.save_and_reraise_exception(logger=LOG):
+                LOG.error("Exception occured: %s", Exception)
                 self._save_blob_info(
                     context, af, field_name, blob_key, None)