Does not halt the GitHub import process when an error occurs

What are the relevant issue numbers?

Fixes #20385

https://gitlab.com/gitlab-org/gitlab-ce/issues/20149

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together