Error message for post approval when topic is closed needs improving

This came up in chat for another site I moderate:

It looks like moderators who try to approve a post in a topic that was closed while the post was in the approval queue get the same message as a regular user who opens the composer prior to a topic being closed. We should give staff a different error message.

The error is correct - the topic is closed - but the error doesn’t tell staff what they need to do.