[DPE-8865] Extract error messages

[marceloneppel]

Issue

Sometimes the error message from pgBackRest is sent to stdout instead of stderr (and we don't log them, as seen in #1280).

Solution

Check both stdout and stderr for errors and warnings. Debugging with the Field Engineering team before the sprint in Gothenburg, I figured out that some errors are reported as warnings (one case is when the replica cannot communicate with the primary through the pgBackRest TLS server because its IP is not in the SANs list, which was already fixed by #1162).

Fixes #1280

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.90%. Comparing base (28b6219) to head (9813b49).

Files with missing lines	Patch %	Lines
src/backups.py	81.25%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1320      +/-   ##
===========================================
+ Coverage    69.82%   69.90%   +0.07%     
===========================================
  Files           16       16              
  Lines         4149     4173      +24     
  Branches       658      664       +6     
===========================================
+ Hits          2897     2917      +20     
- Misses        1045     1048       +3     
- Partials       207      208       +1     

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