Handle error when adding pending contact

Found by @grote in python-briar-wrapper#19 (closed).

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 (closed) 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.