Add exit code log and possible memory shortage warning log for Restic command failure

[blackpiglet]

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Does your change fix a particular issue?

Fixes #6001

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• 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-commenter]

Codecov Report

Merging #6459 (d7f1ea4) into main (84eca51) will increase coverage by 1.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #6459      +/-   ##
==========================================
+ Coverage   59.14%   60.18%   +1.04%     
==========================================
  Files         229      229              
  Lines       24184    24217      +33     
==========================================
+ Hits        14303    14575     +272     
+ Misses       8880     8634     -246     
- Partials     1001     1008       +7     

Impacted Files	Coverage Δ
pkg/restic/exec_commands.go	15.05% <0.00%> (-0.17%)	⬇️

... and 7 files with indirect coverage changes