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

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

app/services/activitypub/process_account_service.rb

@@ -23,6 +23,8 @@ class ActivityPub::ProcessAccountService < BaseService
         create_account if @account.nil?
         update_account
         process_tags
+      else
+        raise Mastodon::RaceConditionError
       end
     end