Draft: common: handle GitlabCreateError when creating group milestones
Fixes Sentry issue where jirahook repeatedly fails trying to create group milestones that already exist at the project level.
When milestone creation fails with "already being used" error, the code now gracefully handles two scenarios:
- Race condition: Another jirahook instance created the milestone at group level - we fetch and return the existing group milestone
- Project-level conflict: A project-level milestone exists with the same name, blocking group-level creation - we log a clear warning message directing admins to delete or promote the project milestone
This prevents the error from being logged to Sentry thousands of times and provides actionable guidance for resolving the underlying conflict.
Signed-off-by: Alexandra Hájková ahajkova@redhat.com