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)