Acceptance test improvement: include the number of feeds that have unparseable rows grouped by file

[emmambd]

As a continuation of work done for #1698 and investigation done for #1767, we want to see in the acceptance tests which feeds have unparseable rows, grouped by file.

This would help us assess how commonly occurring the unparseable error is, and for which files it is most common. It'll also help us address any prospective issues with the new Flex changes.

[jcpitre]

Tasks:

• Determine where in the report to put this information (it might already be there)
  • Decide what language to use for flagging this in the report
• Put it in validation report (json)
• Add to acceptance tests: json and PR comments
  • In the PR comment: summary # of feeds with unparseable rows and files associated with it
  • In json: have a list of feed_id, unparseable_file