Add better error handling

Issue Summary

Probably due to my weird setup described in #78509, the user import failed, and the private message import failed as well (likely because there were no users to import against ;)). The only indication I had of these two failing (aside from empty tables) is I was directed back to the drop-down with no status messages, unlike when the topics and other things imported fine and there was a status message to tell me what happened.

So adding some indication of where the query failed and why would be cool. When poll module's import failed it did that, but only because Drupal spat out an error that got flagged by watchdog.