FIX: `rescue_from` doesn't bubble up.

See a47e0c19e6/actionpack/lib/action_controller/metal/rescue.rb (L25)

app/controllers/application_controller.rb

@@ -124,7 +124,7 @@ class ApplicationController < ActionController::Base
   rescue_from PG::ReadOnlySqlTransaction do |e|
     Discourse.received_postgres_readonly!
     Rails.logger.error("#{e.class} #{e.message}: #{e.backtrace.join("\n")}")
-    raise Discourse::ReadOnly
+    rescue_with_handler(Discourse::ReadOnly.new) || raise
   end
 
   rescue_from ActionController::ParameterMissing do |e|
@@ -212,8 +212,10 @@ class ApplicationController < ActionController::Base
   end
 
   rescue_from Discourse::ReadOnly do
+    unless response_body
       render_json_error I18n.t('read_only_mode_enabled'), type: :read_only, status: 503
     end
+  end
 
   def redirect_with_client_support(url, options)
     if request.xhr?