script/backport-create-issue: retry without if assignee is invalid

script/backport-create-issue: retry without if assignee is invalid

Resolves:

    WARNING:root:Missing issues will be created in Backport tracker of the relevant Redmine project
    INFO:root:Redmine key was read from '$REDMINE_API_KEY'; using it
    INFO:root:Processing 35 issues with status Pending Backport
    Traceback (most recent call last):
      File "/home/runner/work/ceph/ceph/src/script/backport-create-issue", line 411, in <module>
        iterate_over_backports(redmine, issues, dry_run=args.dry_run)
      File "/home/runner/work/ceph/ceph/src/script/backport-create-issue", line 354, in iterate_over_backports
        update_relations(r, issue, dry_run)
      File "/home/runner/work/ceph/ceph/src/script/backport-create-issue", line 242, in update_relations
        other = r.issue.create(project_id=issue['project']['id'],
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.9/x64/lib/python3.11/site-packages/redminelib/managers/base.py", line 187, in create
        response = self.redmine.engine.request(self.resource_class.http_method_create, url, data=request)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.9/x64/lib/python3.11/site-packages/redminelib/engines/base.py", line 83, in request
        return self.process_response(self.session.request(method, url, **kwargs))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/opt/hostedtoolcache/Python/3.11.9/x64/lib/python3.11/site-packages/redminelib/engines/base.py", line 178, in process_response
        raise exceptions.ValidationError(', '.join(': '.join(e) if isinstance(e, list) else e for e in errors))
    redminelib.exceptions.ValidationError: Assignee is invalid
    Examining issue#66797 (1/35)

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>