Error handler can use the label of the ExitCode

[superstar54]

Before saving the error handlers, we convert the ExitCode's label (str) to the ExitCode's status (int).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.19048% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.77%. Comparing base (5937b88) to head (cdd37d8).
Report is 74 commits behind head on main.

Files with missing lines	Patch %	Lines
aiida_workgraph/task.py	70.00%	3 Missing ⚠️
aiida_workgraph/workgraph.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
+ Coverage   75.75%   80.77%   +5.01%     
==========================================
  Files          70       66       -4     
  Lines        4615     5128     +513     
==========================================
+ Hits         3496     4142     +646     
+ Misses       1119      986     -133     

Flag	Coverage Δ
python-3.11	80.69% <76.19%> (+5.02%)	⬆️
python-3.12	80.69% <76.19%> (?)
python-3.9	80.72% <76.19%> (+4.99%)	⬆️

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