Remove the function of messages.error in AddRouterRoute

The function of exceptions.handle can throw an error message,
no need messages.error

Change-Id: I9cd07a314a13ec3118d031d29d88b0d926389c52

openstack_dashboard/dashboards/project/routers/extensions/extraroutes/forms.py

@@ -48,12 +48,10 @@ class AddRouterRoute(forms.SelfHandlingForm):
             LOG.info('Invalid format for routes %(route)s: %(exc)s',
                      {'route': route, 'exc': e})
             msg = _('Invalid format for routes: %s') % e
-            messages.error(request, msg)
             redirect = reverse(self.failure_url, args=[router_id])
             exceptions.handle(request, msg, redirect=redirect)
         except Exception as e:
             LOG.info('Failed to add route: %s', e)
             msg = _('Failed to add route: %s') % e
-            messages.error(request, msg)
             redirect = reverse(self.failure_url, args=[router_id])
             exceptions.handle(request, msg, redirect=redirect)