Fix VPNaaS success messages translation

Success messages for VPNaaS "Delete" options are translatable now.
Also this patch set adds descriptions to error messages.

Fixes bug 1252239

Change-Id: I6e15f7c594ff20dcfe7587a19540d11aaeb97e3b
Corrected incorrect message when deleting port.

Updated the file /horizon/openstack_dashboard/dashboards/admin/
networks/ports/tables.py so it will give the correct error message
when trying to delete a port that is in use.This updated error
message will give the same information as when using the command-line.

Change-Id: I62c0fb1c5b9b073c0d083bddc45133d502b554be
Closes-Bug: #1251314