Adding contacts remotely does not check return value, hides errors

It looks like, the result of the POST request isn't checked here:

https://code.briarproject.org/briar/python-briar-wrapper/-/blob/b8ac26b21dd5b3580d49a582c7b16cebd6c3dc3e/briar_wrapper/models/contacts.py#L37

I found this when debugging why failed pending contacts don't get re-added. Check how the Android UI handles this:

• https://code.briarproject.org/briar/briar/-/blob/1f246637e25a9e53e7b9ec8961e1ad6bf26a7daf/briar-android/src/main/java/org/briarproject/briar/android/contact/add/remote/NicknameFragment.java#L123

• https://code.briarproject.org/briar/briar/-/blob/1f246637e25a9e53e7b9ec8961e1ad6bf26a7daf/briar-android/src/main/java/org/briarproject/briar/android/contact/add/remote/AddContactViewModel.java#L124