DEV: Fix undefined method in rails_failover initializer.

config/initializers/002-rails_failover.rb

@@ -66,12 +66,12 @@ if ENV["ACTIVE_RECORD_RAILS_FAILOVER"]
 
   RailsFailover::ActiveRecord.register_force_reading_role_callback do
     begin
-      Discourse.redis.exists?(
+      Discourse.redis.exists(
         Discourse::PG_READONLY_MODE_KEY,
         Discourse::PG_FORCE_READONLY_MODE_KEY
       )
     rescue => e
-      Discourse.warn_exception(e)
+      Discourse.warn_exception(e, message: "Failed to force AR reading role")
       false
     end
   end