Add better error handling to BulkImports::GroupLoader

What does this MR do and why?

This MR:

• Fixes a bug where an unhadled error during group creation did not update Import state properly, giving an impression the import was completed, where in reality it failed
• Adds a bit of resiliency to BulkImport's GroupLoader, in order to provide better error handling. We want to raise exceptions in the GroupLoader which get bubbled up and handled in the BulkImports::Pipeline::Runner and update the import state properly (http://gitlab.com/gitlab-org/gitlab/blob/georgekoltsov/enhance-group-migration-resiliency/lib/bulk_imports/pipeline/runner.rb#L47-L65)

Mentions https://gitlab.com/gitlab-org/gitlab/-/issues/342522

Screenshots or screen recordings

How to set up and validate locally

1. Feature.enable(:bulk_import)
2. Create a top level group
3. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api scope)
4. Select newly created group, enter destination to be user namespace
5. Wait for Group import to complete and verify it failed
6. Verify bulk_import_failures table contains an error and bulk_import state is failed (-1)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.