conversation_module: add a fallback if the clone fails

This fix testAddAcceptOfflineThenConnects without reverting
7463a8a2
The idea is in case of a sporadic clone failure (peer offline but
present, socket broken while cloning, etc), we schedule a new
clone operation later.

GitLab: #991
Change-Id: Ifacd6bfe383902bff6cb818e41ff8f5ddd0f6917