Add feature for firing group completion callbacks on failure

[MyrikLD]

add_completion_callback never fires if any task in the group fails permanently — the barrier just hangs forever. But a permanent failure is a completion; the group is done, just not successfully. The current behaviour is closer to add_success_callback.
Proposed fix: add fire_on_failure=False to GroupCallbacks. When True, the barrier is decremented on permanent failure (message.failed) and on skipped messages, firing the callbacks regardless of outcome. Defaults to False for backwards compatibility.
Prior art: dramatiq-workflow works around this externally via message.failed — the need is real.
Happy to submit a PR if this direction sounds good.