debug output for skipped rows

Problem/Motivation

It would be nice if there was output of the values for rows which are skipped.

There's MigrateEvents::MAP_SAVE event which gets fired for skipped rows as well as other rows, but it doesn't get the row passed to the event unfortunately -- would need to be added to core first.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes