Our current logic for processing responses from the MultistatementTransactionsRequestSender will just directly return to the user any local errors encountered while trying to send sub-batches to shards.

However, this is not always desirable because:

1. If it's an unordered write, we should still try to complete the rest of the writes following those that failed in this sub-batch
2. If it's an ordered write, we want to report results for any writes that succeeded before the sub-batch errored 

We should update this logic to do something similar to batched writes, which is:

1. if the error is a transient transaction error, return it directly to the user as a top-level error so the client may retry the entire transaction
2. if the error is because the mongos is shutting down,  return it directly to the user as a top-level error so the client may retry the command against another mongos
3. otherwise, record the error as a per-statement error within the bulkWrite cursor (i.e. record it for the first write in the sub-batch if doing ordered writes, or all writes in the sub-batch for unordered)
4. if we are in a transaction, abort execution (even if we are doing an unordered write) since we abort transactions on any errors