fix(vermeer): parse task error_message

[PNHD]

Summary

Parse the error_message field returned by the Vermeer task API into
TaskInfo.

The field is now exposed through the error_message property and included in
to_dict(). Missing fields keep the existing client-style empty-string
default.

Tests

• added coverage for responses with error_message
• added coverage for responses without error_message
• targeted Vermeer tests pass
• targeted Ruff check and format check pass

Closes #371.

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues; targeted tests and checks pass.

Review effort: Lite
Findings: None

What changed in this PR

Adds parsing and serialization of Vermeer task error_message values in TaskInfo.

Changes:

• Exposes error_message with an empty-string default.
• Includes it in to_dict().
• Adds coverage for present and missing values.

File	Description
vermeer-python-client/​src/​tests/​structure/​test_task_data.py	Tests populated and missing error messages.
vermeer-python-client/​src/​pyvermeer/​structure/​task_data.py	Adds error-message support to TaskInfo.

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[imbajin]

Blocking: no. Summary: TaskInfo now preserves the optional Vermeer error_message through parsing and serialization. Evidence: exact-head diff and tests for populated and absent values; latest-head checks passed. Score: 8.7/10.