Updated to_native behaviour: Raise ``ConversionError`` on incorrect ObjectId value

* Catch ``bson.errors.InvalidId`` exception and rerise as ``ConversionError`` during ``to_native``
Merge pull request #1 from miphreal/master

Updated to_native behaviour: Raise ``ConversionError`` on incorrect Obje...
Merge pull request #2 from fixfork/master

[contrib.mongo] Updated to_native behavior