Fix uncaught 422 and 500 errors (#11590)

app/controllers/media_proxy_controller.rb

@@ -7,6 +7,8 @@ class MediaProxyController < ApplicationController
 
   before_action :authenticate_user!, if: :whitelist_mode?
 
+  rescue_from ActiveRecord::RecordInvalid, with: :not_found
+
   def show
     RedisLock.acquire(lock_options) do |lock|
       if lock.acquired?