handle errors and fix join form

app/controllers/errors_controller.rb

@@ -0,0 +1,14 @@
+class ErrorsController < ApplicationController
+  
+  def not_found
+    render status: 404
+  end
+  
+  def unprocessable
+    render status: 422
+  end
+
+  def internal_error
+    render status: 500
+  end
+end