Handle timeout error in background jobs

[tnagorra]

Changes

• Fix serialization of nested errors
• Remove contributorsCount from project type
• Handle timeout issue on background tasks

This PR doesn't introduce any:

• temporary files, auto-generated files or secret keys
• n+1 queries
• flake8 issues
• print
• typos
• unwanted comments

This PR contains valid:

• tests
• permission checks (tests here too)
• translations

[codecov]

Codecov Report

❌ Patch coverage is 43.85965% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.05%. Comparing base (d3aec10) to head (2dff549).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
utils/graphql/drf.py	47.05%	14 Missing and 4 partials ⚠️
project_types/base/tutorial.py	11.11%	8 Missing ⚠️
project_types/base/project.py	14.28%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #192      +/-   ##
===========================================
- Coverage    87.08%   87.05%   -0.04%     
===========================================
  Files          204      204              
  Lines        11807    11841      +34     
  Branches      1004     1013       +9     
===========================================
+ Hits         10282    10308      +26     
- Misses        1141     1146       +5     
- Partials       384      387       +3     

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