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

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

app/controllers/media_proxy_controller.rb

@@ -8,6 +8,8 @@ class MediaProxyController < ApplicationController
       if lock.acquired?
         @media_attachment = MediaAttachment.remote.find(params[:id])
         redownload! if @media_attachment.needs_redownload? && !reject_media?
+      else
+        raise Mastodon::RaceConditionError
       end
     end