Show exceptions to the user on the list of migrations

Problem/Motivation

On pages like admin/structure/migrate/manage/[some migration group]/migrations, if an error occurs when building a row all that's shown is 'No data found' and 'N/A' for the row. No error message is shown or saved. And, if an error occurs getting the migration data, that error message is only saved to the log.

Steps to reproduce

Do something that generates an error. In my case it was using the wordpress_migrate module and not permanently saving the import file. You can, of course, add fake exceptions to the MigrationListBuilder::buildRow method.

Proposed resolution

In both cases, show error messages to the user on the listing page. I removed logging the error getting the migration data, but that could be put back.

If there's a security risk in showing the user details on the exceptions, then the exceptions could be logged and generic error messages would be shown to the user. However, one would think anyone who migrates data would be a top level admin.

Remaining tasks

Test.

User interface changes

Shows error messages to the user.

API changes

None.

Data model changes

None.