Raise Mastodon::RaceConditionError if Redis lock failed (#7511)

An explicit error allows user agents to know the error and Sidekiq to
retry.

app/services/fetch_link_card_service.rb

@@ -23,6 +23,8 @@ class FetchLinkCardService < BaseService
       if lock.acquired?
         @card = PreviewCard.find_by(url: @url)
         process_url if @card.nil? || @card.updated_at <= 2.weeks.ago
+      else
+        raise Mastodon::RaceConditionError
       end
     end