V3: More luxurious error when UAA is unavailable during a POST

v3/domains with a router group

[finishes #162395328]

Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>
Co-authored-by: Mona Mohebbi <mmohebbi@pivotal.io>

Author: reidmit

app/controllers/v3/domains_controller.rb

@@ -218,7 +218,7 @@ def fetch_router_group(router_group_guid)
     routing_client.router_group(router_group_guid)
   rescue RoutingApi::RoutingApiUnavailable
     service_unavailable!('The Routing API is currently unavailable. Please try again later.')
-  rescue RoutingApi::UaaUnavailable
+  rescue UaaUnavailable, RoutingApi::UaaUnavailable
     service_unavailable!('Communicating with the Routing API failed because UAA is currently unavailable. Please try again later.')
   end
 end

spec/request/domains_spec.rb

@@ -830,7 +830,7 @@
         let(:user_header) { admin_headers_for(user) }
 
         before do
-          allow(routing_api_client).to receive(:router_group).and_raise VCAP::CloudController::RoutingApi::UaaUnavailable
+          allow(routing_api_client).to receive(:router_group).and_raise VCAP::CloudController::UaaUnavailable
         end
 
         it 'returns a 503 with a helpful message' do