Do not throw when transitioning to PENDING_DISABLE if index is already disabled

When multiple clients get index write failures, they may attempt to transition to PENDING_DISABLE concurrently. A race condition can cause one to exhaust its retries first and transition the index to DISABLE, and another client to then attempt to transition from DISABLE to PENDING_DISABLE. Currently this is causing an exception to get thrown, leading to the KillServerOnFailurePolicy getting activated.