Provide an overview of successful/failed content after import has finished

It'd be good to be able to tell how many content items were actually imported and how many failed, maybe with a list of canonical ids of the failed posts for further investigation.

No files, tests, or entry points are named in the issue. Start by locating the import completion and failure-handling paths, then determine how imported items and canonical IDs are represented; done means the completed import reports successful and failed counts and lists failed post IDs for investigation.

Written by the indexing model from the issue text.