fix: Set error value from validation exception properly for feature seralizer

[zachaysan]

Thanks for submitting a PR! Please check the boxes below:

• I have run pre-commit to check linting
• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

To access the string value of an exception the proper way is to call str() on the exception itself, not referencing .message so this code updates the call.

How did you test this code?

I manually tested a try and except prototype to mimic the behaviour.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: api/features/serializers.py

Function	Unhandled Issue
save	AttributeError: 'ValidationError' object has no attribute 'message' /api/v1/environments/{environment_api_key}/identities/{identity_pk}/... Event Count: 1
save	AttributeError: 'ValidationError' object has no attribute 'message' /api/v1/features/... Event Count: 1

---

_{Did you find this useful? React with a 👍 or 👎}

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 19, 2024 0:48am
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 19, 2024 0:48am
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 19, 2024 0:48am

[github-actions]

Uffizzi Preview deployment-50494 was deleted.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.93%. Comparing base (e138b4e) to head (895d8fa).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3809   +/-   ##
=======================================
  Coverage   95.93%   95.93%           
=======================================
  Files        1103     1103           
  Lines       34801    34801           
=======================================
  Hits        33386    33386           
  Misses       1415     1415           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.