When redisplaying pages after continuation expiry, include a user visible error message

[jryans]

At the moment, if you submit a dynamic page (e.g. the package creation form) after the continuation has expired, you are sent to same page with all the dynamic data removed (e.g. form values), but there is no message explaining what happened.

We should add some kind of user-visible error message explaining that the page expired when this happens.

[racket-discourse-github-bot]

This issue has been mentioned on Racket Discussions. There might be relevant details there:

https://racket.discourse.group/t/package-server-new-package-workaround/530/12