Bulk api does not publish CommandFailed events when commands fail with network errors

The _BulkWriteContext class published a CommandStartedEvent before attempted to send a command, however it only publishes a corresponding CommandFailedEvent when the command fails with a server error. If the command fails with something else besides an OperationFailure, the failure is not published.

See: https://github.com/mongodb/mongo-python-driver/blob/3.7.1/pymongo/message.py#L919-L928