[CBM] Provide a clear error when restore fails due to locked documents

Problem

Currently, cbbackupmgr restore may not work if a document on the cluster is locked. This can be reproduced by:

1. Taking a backup of a bucket
2. Modifying a document in a collection
3. Lock a document in the same collection
4. While the document is locked, attempt a cbbackupmgr restore with the --force-updates flag

Fix

Unclear, potentially:

• Adding a flag that skips locked documents
• Returning a clearer error message so it's clear why the restore is failing