[feature] Friendly error messages in UI

As a Workflow Developer

I want to see a friendly error message in the error pop-up in the bottom right corner

So that I can more easily understand what is going on.

Technical Details
Catch all exceptions on the backend, especially Jackson ones, and add appropriate messages. Check `GlobalResponseEntityExceptionHandler` and `*ErrorTypes`

Acceptance Criteria
TODO

Definition of Done
TODO

Start by reading GlobalResponseEntityExceptionHandler and the relevant *ErrorTypes, then trace how backend errors reach the error pop-up in the bottom-right corner. Define the missing acceptance criteria before implementation; done should mean exceptions, especially Jackson errors, produce appropriate friendly messages in the UI.

Written by the indexing model from the issue text.