Unexpected skip of following batches after duplicate error

Description

Hello!

I just investigated undocumented behavior of unordered bulk execution.

simple example (Node.js):

// 1. initialize

const bulk = initializeUnorderedBulkOp();

// 2. populating data

bulk.insert(doc1);

bulk.insert(doc2);

bulk.insert(doc3);

...

bulk.insert(doc10000);

// 3. execution

bulk.execute().catch(error => {

   // ignore duplicete errors because this decision works faster then find().upsert().update()

   // yes, it's crutch, but it works :)

   if (error.code === 11000) {

     return;

   }

   throw error;

});

So... Sometimes in the bulk object I see more then single batch (If it was large source in input). And if some batch has any document duplicate unique key - following batches won't be processed.

Right now i got three batches. First of them has 8547 operations. In bulkResult I find 7 errors (duplicates). So after execution the target collection had increased for 8540 documents. Usually I use 'getWriteErrors()' to collect problems but (surprize), operations from second and next batches didn't have errors because hadn't process. 

I tried to use 'continueOnError' flag, but it didn't has any effect.

Now I use 4.2.7 version but abut month ago I had upgraded from 3.x. I dont't remember this problem before upgrade, but after quick analyse of result collections I may think that this problem can be present in previous versions... 

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