Extend GET job endpoint with errorData field

[manuelwedler]

In #2045, we added a new column error_data to our verification_jobs table. It includes any data that is needed to reconstruct an error message from the error_code. We should add this field also to our API, such that we can make use of it in the UI and render the errors in a friendlier way. The errorData can be returned as it is (a json object) from the database. It should be a subfield of error in the API response.