Fix backup log to show error string, not index

[piny940]

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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:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• 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]

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 ⚠️

Additional details and impacted files

@@           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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