rz_core_task_join: Return fine-grained error values

[kazarmy]

Your checklist for this pull request

• [X] I've read the guidelines for contributing to this repository
• [X] I made sure to follow the project's coding style
• [ ] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [X] I've added tests that prove my fix is effective or that my feature works (if possible)
• [ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr makes rz_core_task_join() return fine-grained error values.

Test plan

All builds except the woodpecker build are green.

Closing issues

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