fix(db): only callback with MongoError NODE-1293

[jlord]

If an error occurs within the users callback the callback is called twice. This checks and only returns the callback with an error if it is a MongoError from the server.

Fixes NODE-1293

[mbroadst]

@jlord please include a test for this

[jlord]

The test that was failing after this change turned out to be a bad test that was expecting an error when it shouldn't have and the error that was actually coming through (and making the test pass previously) was an assertion error from the testing library.

@mbroadst I changed the test to expect error to be null because, if I'm understanding strict = false correctly, no error should be returned if there isn't an collection per: https://github.com/mongodb/node-mongodb-native/blob/3.0.0/lib/db.js#L409.