Graceful session expiration handling on Front-End

We need to gracefully handle case, when user have data on the form, that he haven't submitted yet but his session has expired in background. Right now we just redirect user to homepage without any additional message shown in case if he tries to submit that form.

I think we can have session expiration checker running in background (using ajax), which won't extend session lifetime on it's own of course. This way if session is expired then we can actually notify user and don't allow him to submit any forms on the page (except login one of course).