Log DBReferenceError in archive_deleted_rows_for_table

We should add the foreign key constraint integrity error to the warning
since it includes detailed information about the reference table that
caused the constraint failure, which is useful for knowing things are
out of order without having to check your database.

You get something like this now:

2015-10-07 15:29:38.238 WARNING nova.db.sqlalchemy.api
[req-6115aef5-2a82-4d42-bf67-eaab2e8eee63 None None] IntegrityError
 detected when archiving table instances: (pymysql.err.IntegrityError)
 (1451, u'Cannot delete or update a parent row: a foreign key constraint
 fails (`nova`.`instance_actions`, CONSTRAINT
 `fk_instance_actions_instance_uuid` FOREIGN KEY (`instance_uuid`)
 REFERENCES `instances` (`uuid`))') [SQL: u'DELETE FROM instances WHERE
 instances.id in (SELECT T1.id FROM (SELECT instances.id \nFROM
 instances \nWHERE instances.deleted != %s ORDER BY instances.id \n
 LIMIT %s) as T1)'] [parameters: (0, 10)]

Related-Bug: #1183523

Change-Id: I41b10898c6e9cc48da58c4d2daf7df362fc9fc36

nova/db/sqlalchemy/api.py

@@ -6017,12 +6017,13 @@ def archive_deleted_rows_for_table(context, tablename, max_rows):
         with conn.begin():
             conn.execute(insert)
             result_delete = conn.execute(delete_statement)
-    except db_exc.DBReferenceError:
+    except db_exc.DBReferenceError as ex:
         # A foreign key constraint keeps us from deleting some of
         # these rows until we clean up a dependent table.  Just
         # skip this table for now; we'll come back to it later.
-        msg = _("IntegrityError detected when archiving table %s") % tablename
-        LOG.warn(msg)
+        LOG.warn(_LW("IntegrityError detected when archiving table "
+                     "%(tablename)s: %(error)s"),
+                 {'tablename': tablename, 'error': six.text_type(ex)})
         return rows_archived
 
     rows_archived = result_delete.rowcount