It is assumed resharding donor state machine can not re-enter kError state as per this invariant. However, this path is currently possible in the following steps:

1. An unrecoverable error is encountered before the donor transitions to kBlockingWrites.
2. Donor state document is updated with kError.
3. Wait for majority after persisting state document locally fails due to some transient error.
4. A retry of this AsyncTry results in entering _transitionToError a second time and fail this invariant.