feat(handler): update migration_error and use value in import notification

mentioned in commit a306d58a

mentioned in issue #623 (closed)

mentioned in merge request gitlab-org/build/CNG!930 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1594 (merged)

mentioned in commit ee59c489

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1602 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2458 (merged)

mentioned in merge request omnibus-gitlab!6064 (merged)

mentioned in merge request gitlab-development-kit!2503 (merged)