FIX: improve error results for message bus failures

Gemfile.lock

@@ -156,7 +156,7 @@ GEM
     mail (2.6.4)
       mime-types (>= 1.16, < 4)
     memory_profiler (0.9.6)
-    message_bus (2.0.1)
+    message_bus (2.0.2)
       rack (>= 1.1.3)
     metaclass (0.0.4)
     method_source (0.8.2)

config/initializers/004-message_bus.rb

@@ -61,6 +61,14 @@ MessageBus.is_admin_lookup do |env|
   env["__mb"][:is_admin]
 end
 
+MessageBus.on_middleware_error do |env, e|
+  if Discourse::InvalidAccess === e
+    [403, {}, ["Invalid Access"]]
+  elsif RateLimiter::LimitExceeded === e
+    [429, {}, [e.description]]
+  end
+end
+
 MessageBus.on_connect do |site_id|
   RailsMultisite::ConnectionManagement.establish_connection(db: site_id)
 end