Don't blow up when email has no References header

What does this MR do?

Don't blow up when the References header isn't present and the To header doesn't match; raise a normal exception instead.

Are there points in the code the reviewer needs to double check?

Not sure. Should I be using that fixture?

Why was this MR needed?

This is a part of fixing https://gitlab.com/gitlab-org/gitlab-ee/issues/2247.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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