In https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/17883, we
added a feature that would allow a project export to be uploaded and
imported successfully before deleting the old copy of the project.

This overwrite feature would always check whether the user had owner
privileges in the group of the project. If the user only had maintainer
privileges or less--even if an overwrite was not happening--the
import/export would fail with an empty `Projects::ImportService::Error`.

To fix this, we make two changes:

1. Only do the permission check if we actually need to overwrite
the project.
2. If the permission check fails, raise an exception with details.