The bson API will currently return errors defined in the bsoncore package. Errors include:

• bsoncore.ErrNilReader
• bsoncore.ErrEmptyKey
• bsoncore.ErrOutOfBounds
• bsoncore.ErrElementNotFound (from Raw.LookupErr)
• bsoncore.InsufficientBytesError (from Raw.LookupErr)
• bsoncore.InvalidDepthTraversalError (from Raw.LookupErr)

Since bsoncore is an unstable API, we shouldn't require users to use error values from that package to use the stable bson API. Move or replace all bsoncore error values so that the bson API only returns errors defined in the bson package.

Definition of done:

• All bson package APIs do not return any errors defined in the bsoncore package (or any other unstable package).