Don't return 500 error for tags in atom format

Sentry error:
https://sentry.gitlab.net/gitlab/gitlabcom/issues/3152896

**Problem**

When Gitaly is unavailable, tags atom requests return a 500 error.

**Solution**

Return 503 (Service unavailable error) instead

Changelog: fixed

app/controllers/projects/tags_controller.rb

@@ -31,6 +31,7 @@ class Projects::TagsController < Projects::ApplicationController
 
     rescue Gitlab::Git::CommandError => e
       @tags = []
+      @releases = []
       @tags_loading_error = e
     end
 

spec/controllers/projects/tags_controller_spec.rb

@@ -31,6 +31,7 @@ RSpec.describe Projects::TagsController do
           get :index, params: { namespace_id: project.namespace.to_param, project_id: project }, format: format
 
           expect(assigns(:tags)).to eq([])
+          expect(assigns(:releases)).to eq([])
           expect(response).to have_gitlab_http_status(:service_unavailable)
         end
       end