Add a value to the error_code in getPlanningContext

[galou]

Set INVALID_MOTION_PLAN if no context loader is found for the requested planner_id before returning.

Description

error_code.val will be set to a meaningful value rather than being unset in case of early return.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.26%. Comparing base (d98fc1f) to head (a1c66fb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3764      +/-   ##
==========================================
- Coverage   46.28%   46.26%   -0.02%     
==========================================
  Files         726      726              
  Lines       59507    59514       +7     
  Branches     7624     7624              
==========================================
- Hits        27536    27527       -9     
- Misses      31805    31818      +13     
- Partials      166      169       +3     

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