Mongo driver 4 rc fix duplicate error

[aljones15]

1. Fixes the Duplicate check
2. Adds a check for MongoError.

[codecov-commenter]

Codecov Report

Merging #95 (325828f) into mongo-driver-4-rc (5b28284) will increase coverage by 1.42%.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           mongo-driver-4-rc      #95      +/-   ##
=====================================================
+ Coverage              87.08%   88.50%   +1.42%     
=====================================================
  Files                      8        8              
  Lines                    813      870      +57     
=====================================================
+ Hits                     708      770      +62     
+ Misses                   105      100       -5     

Impacted Files	Coverage Δ
lib/exceptions.js	100.00% <100.00%> (ø)
lib/helpers.js	97.10% <100.00%> (+2.06%)	⬆️

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[dlongley]

-1 to adding all these, we should only add what we need (and we haven't needed any of these) -- otherwise it's just more to maintain and test.

[aljones15]

That's a good point. There might be an automated way of adding these though. I'll look into it and if not cut them down to something more manageable. I also think not all of these errors are root errors, many of them are the cause of a MongoError I believe.

[aljones15]

Ok so I looked into this today and basically here is the problem: MongoError seems to be nothing more than an abstract class, then MongoError in turn has about 5-6 errors that derive from it, then those 5-6 errors in turn throw the errors listed above and are base classes for a panoply of different errors. So we can pretty safely remove MongoError, but the issue is the driver throws most of the other errors so we probably do need to ensure we know their names.

[aljones15]

ok so I trimmed this down to just the two errors that are kind of sort of thrown by the driver that are not MongoErrors. Then I added a new function assertMongoError here: 43ab9b0
and here:
94bf529

This method should work as the node mongo driver is now written using typescript so there are a lot of Abstract base classes and inheritance based type checking structures that ultimately result in 3 cases: either you get a MongoError, a WriteError or WriteConcernError. For the most part you only get MongoError, but documents can contain WriteError and MongoError can contain both WriteError or WriteConcernError so there might be edge cases were we're asserting on the cause of a MongoError or on the writeErrors property of MongoError or document.