Currently it is fairly complicated to check if a write succeeded or not.
We don't throw an exception on writeerror nor writeconcernerror - but we do on connection and other related errors.

Furthermore, the user has to check both Result->getWriteErrors() and Result->getWriteConcernError() to see if there was an error.
This doesn't make whole lot of sense :/

We should throw MongoDB\Driver\BulkWriteException on bulkWrite failure, which contains ->getWriteResult() which returns the original WriteResult object.
The WriteResult object then has ->getWriteErrors() and ->getWriteConcernError().

For execute[Insert|Update|Delete]() we unwrap the BulkWriteException and throw DuplicateKeyException, WriteConcernException or generic WriteException on failure.