Better error handling for --open_pr

[klieret]

Closes #237

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 72.40%. Comparing base (9da9701) to head (0de54b5).

Files	Patch %	Lines
sweagent/environment/swe_env.py	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
- Coverage   72.52%   72.40%   -0.13%     
==========================================
  Files          17       17              
  Lines        2515     2522       +7     
==========================================
+ Hits         1824     1826       +2     
- Misses        691      696       +5     

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