Fix import/export merge requests error

When MWBS is set and the MR exported/imported an error gets thrown. We should ignore this so it can succeed as the merge user may no longer exist in the target instance.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24009 and https://gitlab.com/gitlab-com/support-forum/issues/1345

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together