[24.0] Revert unnecessary error change

[mvdbeek]

Upgrade messages are collected via step.upgrade_messages. Addresses https://github.com/galaxyproject/galaxy/pull/17858/files#r1557810107

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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[nsoranzo]

Is the content of step.upgrade_messages included somewhere in the dict returned by _workflow_to_dict_export() (if that was the intention)?

[mvdbeek]

It is in the various export flavors, but not systematically.