Track gists import failure

The following discussion from !103761 (merged) should be addressed:

If an object fails to be imported in GitHub Importer - we track such failure in the database. We should consider adding something similar here in order to present any failed imports back to the user. I understand that Gitlab::Import::ImportFailureService has a dependency on project which we do not have here, but perhaps we should still create a follow up for this in order to not forget about it

In the GitHub import, we are tracking critical failures in DB, failing import immediately and presenting this error to the user. For the Gists import, we are logging all errors to importer.log and the only status of overall bulk import is stored in the Redis.

Proposal

Extend import_failures table to add snippet_id/import_id/user_id of sorts to associate gist import and start storing it there.

Edited by Kristina Doskich