RevisionCompactedError need more information

If I use one watch callback function to handle multiple keys，i can not known which watch_id or key is canceled.
Now you must one watch callback function handle one key

Start by locating the watch callback API and the RevisionCompactedError definition in the Python etcd client. Trace how a callback serving multiple keys is notified of cancellation, then make the completed behavior identify the canceled watch ID or key and verify it with the relevant watch tests.

Written by the indexing model from the issue text.