Expose (some) error message if worker fails

[philippotto]

Currently, the user is only informed that a worker job failed, but the reason is not stated. We should discuss how we can improve this. We should not expose the complete stack trace from the worker.

Ideas:

• if a special SpecialError (maybe ValidationError?) is raised from the worker, the message should be exposed
• process worker error automatically (e.g., extract sth via wildcards?)

cc @fm3 @normanrz