Show custom message when tag deletion fails due to import

Proposal

In #349747 (closed) we prevented tags from being deleted when container repositories are being imported as part of the Phase 2 migration.

In the UI, when a user tries to delete a tag while the repository is importing, they see a generic failure message:

Screen_Shot_2022-02-04_at_7.31.29_PM

We should update to show a more specialized message in these cases. Proposed copy:

Tags temporarily cannot be marked for deletion. Please try again in a few minutes. See https://gitlab.com/groups/gitlab-org/-/epics/5523 for more details.