Handle error when adding pending contact
We should definitely fix this in the next release. For now, all we seem to be able to do is tell users about the failure. Once briar#1825 is done, we can offer ways to fix the error.
When implementing this issue, we have to keep in mind that attackers might want to know if a user already has a contact, i.e. exploring the contacts relationships of the targeted user.
In general Briar GTK should start to properly handle errors. When doing this, the documentation of Briar Headless should be improved, too.