WIP: preserve Issue #479 changes

backend/app/controllers/theatres_controller.rb

@@ -55,7 +55,7 @@ class TheatresController < ApplicationController
     theatre = Theatre.find_by(id: params[:id])
     return head :not_found unless theatre
     requested_post_id = params[:post_id].to_i
-    return head :unprocessable_entity if requested_post_id <= 0
+    return head :unprocessable_content if requested_post_id <= 0
 
     skipped = false
     conflicted = false
@@ -95,7 +95,7 @@ class TheatresController < ApplicationController
     theatre = Theatre.find_by(id: params[:id])
     return head :not_found unless theatre
     requested_post_id = params[:post_id].to_i
-    return head :unprocessable_entity if requested_post_id <= 0
+    return head :unprocessable_content if requested_post_id <= 0
 
     conflicted = false