fix(handlers): clarify log message when registry is at max concurrent imports

mentioned in commit 6cbb9ea5

mentioned in merge request gitlab-org/build/CNG!1004 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1752 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1753 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2538 (merged)

mentioned in merge request omnibus-gitlab!6181 (merged)

mentioned in merge request gitlab-development-kit!2656 (merged)