[Worker] Enqueue should return proper error on conflict

[spy16]

Describe the bug
When worker.Enqueue() is called with a Job with an already existing ID, it simply returns the internal duplication error from the queue implementation (e.g., unique constraint

To Reproduce
Steps to reproduce the behavior:

1. Enqueue a job with id x
2. Enqueue another job with id x.

Expected behavior
A predefined error to indicate Conflict should be returned.