give users a nice error message for handles that have expired

[lddubeau]

Version: commit ea54b58

Steps to reproduce

1. Log in.
2. Create a new article. Save it.
3. Invalidate the session.
4. Reload the page with the new article. (Make sure it is accessed through its handle.)

Actual results

A cryptic error message. (In debugging mode, that's a whole trace. A production server would presumably emit something less extensive but just as cryptic for the user.)

Expected results

An error message that explains what is going on and how to recover.

[lddubeau]

In practice nobody has run into this issue so far. So it is not high priority.