SQL Server Management Studio 17.1 Mirroring bug
The previous Connect item has been re-created in the Azure Feedback forum as:


SSMS 17.2 (and 17.1) Mirror configuration fails because mirror database is restoring


If you are experiencing this issue, you may wish to Upvote the Azure item.

Also note that the problem shows up in the recently released SSMS 17.2, 17.4 and even 17.9 as well.