Handle ajax errors
Handle ajax errors

Handle ajax errors according to the circumstance.

* get_user_stack_status: display error to the user, and let them retry
  manually

* get_check_status: display error to the user, let them retry manually

* keepalive: just reschedule

arbrandes/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock