catch BackendError raised by Store(), see #8373

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.54%. Comparing base (1f866ce) to head (628990d). Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/repository.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8420 +/- ## ========================================== - Coverage 81.55% 81.54% -0.02% ========================================== Files 70 70 Lines 12775 12779 +4 Branches 2317 2317 ========================================== + Hits 10419 10420 +1 - Misses 1699 1701 +2 - Partials 657 658 +1 ```

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