Add stoplight for object storage failures, return HTTP 503 (#13043)

app/lib/activitypub/activity/create.rb

@@ -228,6 +228,8 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     emoji ||= CustomEmoji.new(domain: @account.domain, shortcode: shortcode, uri: uri)
     emoji.image_remote_url = image_url
     emoji.save
+  rescue Seahorse::Client::NetworkingError
+    nil
   end
 
   def process_attachments
@@ -250,6 +252,8 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
         media_attachment.save
       rescue Mastodon::UnexpectedResponseError, HTTP::TimeoutError, HTTP::ConnectionError, OpenSSL::SSL::SSLError
         RedownloadMediaWorker.perform_in(rand(30..600).seconds, media_attachment.id)
+      rescue Seahorse::Client::NetworkingError
+        nil
       end
     end