Fix when-all buggy error handling

[acroca]

when using a when_all and one of the tasks fail, the resulting task sometimes failed, sometimes didn't.

[sicoyle]

thank you!!

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.46%. Comparing base (bffb749) to head (2fac29d).
⚠️ Report is 102 commits behind head on main.

Files with missing lines	Patch %	Lines
...w/tests/durabletask/test_orchestration_executor.py	91.30%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
- Coverage   86.63%   80.46%   -6.17%     
==========================================
  Files          84      136      +52     
  Lines        4473    12678    +8205     
==========================================
+ Hits         3875    10201    +6326     
- Misses        598     2477    +1879     

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