Log when creating a VLAN failed, displaying the switch's error

Change-Id: Ia2af415eaf5146d81f5b7158af2db3bd68b0ffd1
diff --git a/util.py b/util.py
index 6bbbd5e..ce41f0c 100644
--- a/util.py
+++ b/util.py
@@ -280,7 +280,8 @@
                     s.switch_disconnect()
                     del s
 
-                except IOError:
+                except IOError as e:
+                    logging.error('Failed to add VLAN %d to switch ID %d (%s): %s', tag, switch.switch_id, switch.name)
                     raise
 
         except IOError: