GitLab Migration - Group Wiki Migration can fail with an exception

Summary

When performing GitLab Migration we import both project and group wiki repositories. For groups that do not have wiki repositories an exception is raised during import:

Gitlab::Git::CommandError 2:fetch remote: "remote: A repository for this group wiki does not exist yet.\nfatal: repository 'http://gdk.test:3000/mygroup/testing-group.wiki.git/' not found\n": exit status 128.

Steps to reproduce

1. Use GitLab Migration for a group that does not have wiki
2. Check bulk_import_failures table to see the error

What is the current bug behavior?

Exception is raised

What is the expected correct behavior?

We should not raise an error in this case. Skip such group wiki if it's not present.