Add error message when migrate from distributed router to centralized

When trying to migrate from distributed router to centralized
a NotImplementedError occurs with empty message.
Need to concretize a message.

Closes-Bug: #1477509
Change-Id: I2b57ec5af38a0f87c1ab256cb8af66bc443be824

neutron/common/exceptions.py

@@ -69,6 +69,10 @@ class ServiceUnavailable(NeutronException):
     message = _("The service is unavailable")
 
 
+class NotSupported(NeutronException):
+    message = _('Not supported: %(msg)s')
+
+
 class AdminRequired(NotAuthorized):
     message = _("User does not have admin privileges: %(reason)s")
 

neutron/db/l3_dvr_db.py

@@ -87,7 +87,8 @@ class L3_NAT_with_dvr_db_mixin(l3_db.L3_NAT_db_mixin,
             router_res.get('distributed') is False):
             LOG.info(_LI("Centralizing distributed router %s "
                          "is not supported"), router_db['id'])
-            raise NotImplementedError()
+            raise n_exc.NotSupported(msg=_("Migration from distributed router "
+                                           "to centralized"))
         elif (not router_db.extra_attributes.distributed and
               router_res.get('distributed')):
             # Notify advanced services of the imminent state transition

neutron/tests/unit/db/test_l3_dvr_db.py

@@ -85,7 +85,7 @@ class L3DvrTestCase(testlib_api.SqlTestCase):
             'distributed': True
         }
         router_db = self._create_router(router)
-        self.assertRaises(NotImplementedError,
+        self.assertRaises(exceptions.NotSupported,
                           self.mixin._validate_router_migration,
                           self.ctx, router_db, {'distributed': False})