Provide better error descriptions for the new background indexing functionality

Summary:

The current error displayed for background indexing doesn't always accurately represent the problem with the JIRA instance.

Steps to Reproduce:

1. Create a JIRA issue.
2. Add a few comments.
3. Delete the JIRA issue from jiraissue. The comments will be orphaned.
4. Go to the re-index page.

Expected Results:

JIRA would either handle the orphaned records (in this example, comments) or display a different error.

Actual Results:

The error is displayed, as in the attached screenshot.

Notes:

This can be fixed with modification to the database, as in our How to Fix the JIRA is Unable to Perform a Background Re-Index at This Time Error KB.