External Task Client doesn't pass on error message

Steps To Reproduce:
1. Start the attached workflow with a single external task.
2. Run the external task client code attached.
3. Go to the definition view, tab incidents

Problem:
The error message shown isn't the expected "some failure message" but 'Message is null.'

Hint:

• The problem exists for camunda bpm 7.9. and earlier versions (checked 7.8. already)
• The payload provided by the client looks like this:

  { json: true,
        body: { errorMessage: 'some error message', workerId: 'someWorker' } }