Fix backup log to show error string, not index

[piny940]

Thank you for contributing to Velero!

Please add a summary of your change

There was a mistake in golang range. Since errs's type is []string in this case, the code should be:

for _, err := range errs {
...
}

Does your change fix a particular issue?

No issue, but this is related to discussion #7800

Please indicate you've done the following:

• [x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [x] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ ] Updated the corresponding documentation in site/content/docs/main.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 58.67%. Comparing base (f654188) to head (059effc).

Files	Patch %	Lines
pkg/controller/backup_controller.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7805 +/- ## ======================================= Coverage 58.67% 58.67% ======================================= Files 345 345 Lines 28743 28743 ======================================= Hits 16866 16866 Misses 10448 10448 Partials 1429 1429 ```

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