Fix exception message on file not found for backups

What does this MR do and why?

• Fix issue where exception message would cause migration to fail.

How to set up and validate locally

• on 4.4.0 set a backup to a directory that doesn't exist.
• Try to migrate up to 4.4.1

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
• Have you applied correct labels?