fix: Error/CommandError have a output format for 1 argument

[ThomasWaldmann]

(backport from master, only thing todo here was that one place)

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (97fe48c) 82.61% compared to head (420ef45) 82.63%.
Report is 9 commits behind head on 1.4-maint.

Files	Patch %	Lines
src/borg/archiver.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8108      +/-   ##
=============================================
+ Coverage      82.61%   82.63%   +0.01%     
=============================================
  Files             38       38              
  Lines          10966    10966              
  Branches        2098     2098              
=============================================
+ Hits            9060     9062       +2     
+ Misses          1344     1343       -1     
+ Partials         562      561       -1     

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