catch BackendError raised by Store(), see #8373

[ThomasWaldmann]

No description provided.

[codecov]

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 ⚠️

Additional details and impacted files

@@            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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