### What?
If an error occurs while unpublishing a document in the edit view UI,
the toast which shows the error message now displays the actual message
which is sent from the server, if available.
### Why?
Only a generic error message was shown if an unpublish operation failed.
Some errors might be solvable by the user, so that there is value in
showing the actual, actionable error message instead of a generic one.
### How?
The server response is parsed for error message if an unpublish
operation fails and displayed in the toast, instead of the generic error
message.
