Include admin email in token refresh error messages

Address the feedback in https://github.com/hypothesis/lms/pull/6290 about providing a more specific error message that indicates the email address of the admin user we are using, when refreshing the admin token fails:

> I think the error messages need to be more specific. The last thing we want is for the feature to stop working for the whole school and to brick-wall users with an overly-generic error message that doesn't give them the information they need to fix the problem.

See also [this thread](https://hypothes-is.slack.com/archives/C4K6M7P5E/p1716912844542169?thread_ts=1716896133.623599&cid=C4K6M7P5E) in Slack about issues seanh ran into while tesitng this.