Fix uncaught parameter missing exceptions and missing error templates (#11702)

app/controllers/application_controller.rb

@@ -21,11 +21,13 @@ class ApplicationController < ActionController::Base
   helper_method :whitelist_mode?
 
   rescue_from ActionController::RoutingError, with: :not_found
-  rescue_from ActiveRecord::RecordNotFound, with: :not_found
   rescue_from ActionController::InvalidAuthenticityToken, with: :unprocessable_entity
   rescue_from ActionController::UnknownFormat, with: :not_acceptable
+  rescue_from ActionController::ParameterMissing, with: :bad_request
+  rescue_from ActiveRecord::RecordNotFound, with: :not_found
   rescue_from Mastodon::NotPermittedError, with: :forbidden
   rescue_from HTTP::Error, OpenSSL::SSL::SSLError, with: :internal_server_error
+  rescue_from Mastodon::RaceConditionError, with: :service_unavailable
 
   before_action :store_current_location, except: :raise_not_found, unless: :devise_controller?
   before_action :require_functional!, if: :user_signed_in?
@@ -96,10 +98,18 @@ class ApplicationController < ActionController::Base
     respond_with_error(406)
   end
 
+  def bad_request
+    respond_with_error(400)
+  end
+
   def internal_server_error
     respond_with_error(500)
   end
 
+  def service_unavailable
+    respond_with_error(503)
+  end
+
   def single_user_mode?
     @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.where('id > 0').exists?
   end