PyMongo doesn't properly handle some error objects from mongos

This is causing test_drop_indexes_non_existant and test_error_code (both in test_collection.py) to fail when run against mongos.

In the case of test_drop_indexes_non_existant helpers._check_command_response isn't checking allowable_errors against the 'raw' dict. In the case of test_error_code Connection._check_response_to_last_error isn't getting the error code out of the 'errObjects' dict.