STAGING_ERROR column information

Staging library now returns the list of columns involved in staging errors.

This adds a new field to the Error class.

List<String> _columns

It is set in two situations:

1. If there is a match to an ERROR cell, the list will contain a single element which represents the column property of the endpoint.
2. If there is no match found in any row (i.e. MATCH_NOT_FOUND), all the endpoint column properties will be added to the list.