Fix Wiki Pipeline network error if group wiki does not exist

What does this MR do and why?

This MR:

• Fixes a bug when importing project/group wikis as part of GitLab Group Migration (that includes projects) https://docs.gitlab.com/ee/user/group/import/ Bug occurred when groups or projects do not have any wiki pages and exception was raised 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.
• This MR adds a preliminary check on source to see if there are any wiki pages available before doing wiki repository import
• This MR also updates BulkImports::Clients::HTTP to have a more detailed error message when a non-successful http response is returned. Before it was just BulkImports::NetworkError, now it's going to be Unsuccessful response 404 from /api/v4/.../wikis to bring more information on what went wrong.

Mentions #345923 (closed)

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How to set up and validate locally

1. Feature.enable(:bulk_import).
2. Feature.enable(:bulk_import_projects).
3. Create a top level group and project in it.
4. Do not add any wikis to the project/group
5. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api & read_repository scope).
6. Select newly created group and click Import.
7. Wait for Group import to complete and verify bulk_import_failures table. It should not contain any errors from WikiPipeline
8. Add any wikis to project/group and rerun import. Verify that wikis are migrated.

MR acceptance checklist

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