When a replication operation (index/delete/update) fails to be executed properly, we fail the replica and allow master to allocate a new copy of it. At the moment, the node hosting the primary shard is responsible of notifying the master of a failed replica. However, if the replica shard is initializing (`POST_RECOVERY` state), we have a racing condition between the failed shard message and moving the shard into the `STARTED` state. If the latter happen first, master will fail to resolve the fail shard message.

This PR builds on elastic#5800 and fails the engine of the replica shard if a replication operation fails. This protects us against the above as the shard will reject the `STARTED` command from master. It also makes us more resilient to other racing conditions in this area.

…it contained is passed on