Be more specific about errors when adding pending contact

WIP because I still need to test these changes in reality, i.e. build a jar and play around with it.

WIP because I might need to add more information for deleting a pending contact in the client.

Following the docs at https://code.briarproject.org/briar/briar/-/blob/beta-1.2.14/bramble-api/src/main/java/org/briarproject/bramble/api/contact/ContactManager.java#L110

I tested it while implementing error handling in Briar GTK (briar-gtk!97 (merged)).

Related to python-briar-wrapper!21 (merged).

Fixes #1825 (closed)