fix(store): raise error when upload processing status is 'error'

[Tejas-Raj01]

Context:
Currently, when running snapcraft upload --release, the CLI polls the store API for the upload processing status. If the store's automated review fails, the API sometimes returns a status with code: "error" but an empty errors list. Because the existing logic only raised an exception if the errors list was populated, the CLI silently bypassed the error, broke out of the polling loop, and returned a revision number—falsely reporting a successful release to the user when it was actually rejected.

Changes in this PR:

• Updated the notify_upload method in snapcraft/store/client.py to explicitly check if status.get("code") == "error". If this condition is met, the CLI now properly aborts the process and raises a SnapcraftError.
• Added a corresponding unit test test_notify_upload_error_status_raises_error in tests/unit/store/test_client.py to ensure regressions are prevented.

Fixes #6299

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[Tejas-Raj01]

Hi @mr-cal,

The PR is ready and all the relevant checks (lint, fast tests, slow tests) have passed successfully. The spread test failures appear to be unrelated infrastructure issues.

Could you please review this when you have a moment? Thanks!

[Tejas-Raj01]

Hi @mr-cal,

Just a quick follow-up on this PR. All relevant unit tests and lint checks have passed successfully. The failing spread tests appear to be caused by unrelated infrastructure issues on the Ubuntu 26.04 runners.

Could you please review the changes when you have a chance? Thanks!